Re: Preferred orientation and lattice parameters
How much mismatch? If you're getting a good profile fit, I'd be inclined to doubt that it is a consequence of preferred orientation. You could mix your sample with another material (corundum powder, cork) to try to reduce the degree of preferred orientation, and see if that makes a difference. Instead of a Le Bail fit, you could try a full Rietveld to see if the powder really is the same phase as the single crystal. One could also imagine that a powder would be more facile in exchanging solvent with the environment, which could affect lattice parameters. On Wed, Feb 5, 2014 at 1:37 PM, Breogan Pato Doldan breogan.p...@udc.eswrote: Dear rietvelders, I have carried out the single crystal resolution and the PXRD Le Bail refinement of a Metal Organic Framework. The compound crystalizes as needles and there is a clear in-plane preferred orientation along the c-axis. There is a mismatch between the single crystal and the Le Bail refinement data in the lattice parameter c. Could this mismatch be due to the preferred orientations? Regards, Breogán Pato Doldán, Fundamental Chemistry Department. Faculty of Sciences. University of A Coruña Rúa da Fraga 10, 15008, A Coruña. Spain. Tel: +34 981167000 ext. 2061 ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++ -- ** Peter W. Stephens Professor, Department of Physics and Astronomy Associate Dean for Curriculum, College of Arts and Sciences Stony Brook University (631) 632-8156 http://mini.physics.sunysb.edu/~pstephens Please update your records to my new email: peter.steph...@stonybrook.edu ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++
Re: Preferred orientation and lattice parameters
I would like to thank you both. It is a small mismatch (about 0.25 A). Due to the structure of the compound and its porosity it really makes sense that the exchange of the guest inside the cavity can affect the lattice. Regards - Mensaje original - De: Peter Stephens peter.steph...@stonybrook.edu Para: Breogan Pato Doldan breogan.p...@udc.es, rietveld_l@ill.fr Rietveld_L@ill.fr Enviados: Miércoles, 5 de Febrero 2014 11:55:59 Asunto: Re: Preferred orientation and lattice parameters How much mismatch? If you're getting a good profile fit, I'd be inclined to doubt that it is a consequence of preferred orientation. You could mix your sample with another material (corundum powder, cork) to try to reduce the degree of preferred orientation, and see if that makes a difference. Instead of a Le Bail fit, you could try a full Rietveld to see if the powder really is the same phase as the single crystal. One could also imagine that a powder would be more facile in exchanging solvent with the environment, which could affect lattice parameters. On Wed, Feb 5, 2014 at 1:37 PM, Breogan Pato Doldan breogan.p...@udc.es wrote: Dear rietvelders, I have carried out the single crystal resolution and the PXRD Le Bail refinement of a Metal Organic Framework. The compound crystalizes as needles and there is a clear in-plane preferred orientation along the c-axis. There is a mismatch between the single crystal and the Le Bail refinement data in the lattice parameter c. Could this mismatch be due to the preferred orientations? Regards, Breogán Pato Doldán, Fundamental Chemistry Department. Faculty of Sciences. University of A Coruña Rúa da Fraga 10, 15008, A Coruña. Spain. Tel: +34 981167000 ext. 2061 ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++ -- ** Peter W. Stephens Professor, Department of Physics and Astronomy Associate Dean for Curriculum, College of Arts and Sciences Stony Brook University (631) 632-8156 http://mini.physics.sunysb.edu/~pstephens Please update your records to my new email: peter.steph...@stonybrook.edu ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++
Re: Preferred orientation and lattice parameters
Hi! Be careful with LeBail. There is a paper (I can send it to you) showing, that this method may give some errors (so is probably also for Pawley method). Best regards! Btw: preferred orientation should not influence the c parameter, but the intensity of an reflection. It can be influenced by sample displacement or (detector or XRD plane) zero shift, and of course due to impurities. Try Rietveld. Good luck! L. Kruszewski -- Łukasz Kruszewski, Ph.D., adjunct Polish Academy of Sciences Institute of Geological Sciences X-Ray Diffraction Laboratory (coordinator) Twarda 51/55 str. 00-818 Warsaw Poland Dear rietvelders, I have carried out the single crystal resolution and the PXRD Le Bail refinement of a Metal Organic Framework. The compound crystalizes as needles and there is a clear in-plane preferred orientation along the c-axis. There is a mismatch between the single crystal and the Le Bail refinement data in the lattice parameter c. Could this mismatch be due to the preferred orientations? Regards, Breogán Pato Doldán, Fundamental Chemistry Department. Faculty of Sciences. University of A Coruña Rúa da Fraga 10, 15008, A Coruña. Spain. Tel: +34 981167000 ext. 2061 ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++
Re: Preferred orientation and lattice parameters
Dear Breogan, just three small additions: (i) .25 Å is a lot, if the lattice parameter in that direction is 4 Å, but much less if it is 40 Å (ii) Definitely, as Peter indicated, partial exchange of guest molecules could be responsible for the differences. This could then be indicated by a broadening of the peaks . (iii) If there is a texture as you have described, one could also consider the possibility that differently oriented crystals in your specimen have different lattice parameters. That is (effectively) the case in the case of residual macrostress, but may be also conceivable (as a slightly exotic case) in the case composition variations This could be checked by measuring the lattice parameters in transmission and reflection geometry. Best regards Andreas On 05.02.2014 20:11, Breogan Pato Doldan wrote: I would like to thank you both. It is a small mismatch (about 0.25 A). Due to the structure of the compound and its porosity it really makes sense that the exchange of the guest inside the cavity can affect the lattice. Regards *De: *Peter Stephens peter.steph...@stonybrook.edu *Para: *Breogan Pato Doldan breogan.p...@udc.es, rietveld_l@ill.fr Rietveld_L@ill.fr *Enviados: *Miércoles, 5 de Febrero 2014 11:55:59 *Asunto: *Re: Preferred orientation and lattice parameters How much mismatch? If you're getting a good profile fit, I'd be inclined to doubt that it is a consequence of preferred orientation. You could mix your sample with another material (corundum powder, cork) to try to reduce the degree of preferred orientation, and see if that makes a difference. Instead of a Le Bail fit, you could try a full Rietveld to see if the powder really is the same phase as the single crystal. One could also imagine that a powder would be more facile in exchanging solvent with the environment, which could affect lattice parameters. On Wed, Feb 5, 2014 at 1:37 PM, Breogan Pato Doldan breogan.p...@udc.es mailto:breogan.p...@udc.es wrote: Dear rietvelders, I have carried out the single crystal resolution and the PXRD Le Bail refinement of a Metal Organic Framework. The compound crystalizes as needles and there is a clear in-plane preferred orientation along the c-axis. There is a mismatch between the single crystal and the Le Bail refinement data in the lattice parameter c. Could this mismatch be due to the preferred orientations? Regards, Breogán Pato Doldán, Fundamental Chemistry Department. Faculty of Sciences. University of A Coruña Rúa da Fraga 10, 15008, A Coruña. Spain. Tel: +34 981167000 ext. 2061 tel:%2B34%20981167000%20ext.%202061 ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr mailto:lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++ -- ** Peter W. Stephens Professor, Department of Physics and Astronomy Associate Dean for Curriculum, College of Arts and Sciences Stony Brook University (631) 632-8156 http://mini.physics.sunysb.edu/~pstephens http://mini.physics.sunysb.edu/%7Epstephens Please update your records to my new email: peter.steph...@stonybrook.edu mailto:peter.steph...@stonybrook.edu -- Dr. Andreas Leineweber Max-Planck-Institut fuer Intelligente Systeme (ehemals Max-Planck-Institut fuer Metallforschung) Heisenbergstrasse 3 70569 Stuttgart Germany Tel. +49 711 689 3365 Fax. +49 711 689 3312 e-mail: a.leinewe...@mf.mpg.de home page of department: http://www.is.mpg.de/de/mittemeijer ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++
RE: Preferred orientation and lattice parameters [SEC=UNCLASSIFIED]
Dear Breogan, It could also come from the flexibility of the MOF which is really common in this class of materials (it can reach 230% in some case). Was the single crystal and PXRD measurement performed in the same condition (temperature, pressure and same solvent within the pores)? To get a better understanding, you can perform PXRD on the solid but in different conditions (ex: as-synthesized, dried, and different solvent) or even better a thermo X-ray diffraction. Regards Hub From: rietveld_l-requ...@ill.fr [mailto:rietveld_l-requ...@ill.fr] On Behalf Of Andreas Leineweber Sent: Thursday, 6 February 2014 5:25 PM To: Breogan Pato Doldan; rietveld_l@ill.fr Subject: Re: Preferred orientation and lattice parameters Dear Breogan, just three small additions: (i) .25 Å is a lot, if the lattice parameter in that direction is 4 Å, but much less if it is 40 Å (ii) Definitely, as Peter indicated, partial exchange of guest molecules could be responsible for the differences. This could then be indicated by a broadening of the peaks . (iii) If there is a texture as you have described, one could also consider the possibility that differently oriented crystals in your specimen have different lattice parameters. That is (effectively) the case in the case of residual macrostress, but may be also conceivable (as a slightly exotic case) in the case composition variations This could be checked by measuring the lattice parameters in transmission and reflection geometry. Best regards Andreas On 05.02.2014 20:11, Breogan Pato Doldan wrote: I would like to thank you both. It is a small mismatch (about 0.25 A). Due to the structure of the compound and its porosity it really makes sense that the exchange of the guest inside the cavity can affect the lattice. Regards De: Peter Stephens peter.steph...@stonybrook.edumailto:peter.steph...@stonybrook.edu Para: Breogan Pato Doldan breogan.p...@udc.esmailto:breogan.p...@udc.es, rietveld_l@ill.frmailto:rietveld_l@ill.fr Rietveld_L@ill.frmailto:Rietveld_L@ill.fr Enviados: Miércoles, 5 de Febrero 2014 11:55:59 Asunto: Re: Preferred orientation and lattice parameters How much mismatch? If you're getting a good profile fit, I'd be inclined to doubt that it is a consequence of preferred orientation. You could mix your sample with another material (corundum powder, cork) to try to reduce the degree of preferred orientation, and see if that makes a difference. Instead of a Le Bail fit, you could try a full Rietveld to see if the powder really is the same phase as the single crystal. One could also imagine that a powder would be more facile in exchanging solvent with the environment, which could affect lattice parameters. On Wed, Feb 5, 2014 at 1:37 PM, Breogan Pato Doldan breogan.p...@udc.esmailto:breogan.p...@udc.es wrote: Dear rietvelders, I have carried out the single crystal resolution and the PXRD Le Bail refinement of a Metal Organic Framework. The compound crystalizes as needles and there is a clear in-plane preferred orientation along the c-axis. There is a mismatch between the single crystal and the Le Bail refinement data in the lattice parameter c. Could this mismatch be due to the preferred orientations? Regards, Breogán Pato Doldán, Fundamental Chemistry Department. Faculty of Sciences. University of A Coruña Rúa da Fraga 10, 15008, A Coruña. Spain. Tel: +34 981167000 ext. 2061tel:%2B34%20981167000%20ext.%202061 ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.commailto:alan.he...@neutronoptics.com Send commands to lists...@ill.frmailto:lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++ -- ** Peter W. Stephens Professor, Department of Physics and Astronomy Associate Dean for Curriculum, College of Arts and Sciences Stony Brook University (631) 632-8156 http://mini.physics.sunysb.edu/~pstephenshttp://mini.physics.sunysb.edu/%7Epstephens Please update your records to my new email: peter.steph...@stonybrook.edumailto:peter.steph...@stonybrook.edu -- Dr. Andreas Leineweber Max-Planck-Institut fuer Intelligente Systeme (ehemals Max-Planck-Institut fuer Metallforschung) Heisenbergstrasse 3 70569 Stuttgart Germany Tel. +49 711 689 3365 Fax. +49 711 689 3312 e-mail: a.leinewe...@mf.mpg.demailto:a.leinewe...@mf.mpg.de home page of department: http://www.is.mpg.de/de/mittemeijer ++ Please do NOT attach files to the whole list alan.he...@neutronoptics.com Send commands to lists...@ill.fr eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++
Re: Preferred orientation question
Dear Ross, I presume on the flat sample you measured regular theta-2theta diagrams, then only probed those planes parallel to the sample plane. In such a situation, you do not probe correctly the texture, and all the models you could envisage are just giving back parameters that have been refined on non-physic grounds. Harmonics will render a very nice fit, just adding on the number of parameters in the development. Measuring the full spectra in order to resolve structural parameters of a textured sample requires that you acquire data in more than few sample orientations, then the quantitative textural information brings orientational physics in the Rietveld fits (http://www.ecole.ensicaen.fr/~chateign/texture/combined.pdf). For this you need to put the sample in a 4-circle diffractometer. cheers daniel Ross H Colman a écrit : Dear All, Could anybody help me with a question I have related to preferred orientation? I have collected some spectra of a sample, on a D4 using flat plate geometry, that I suspect is showing preferred orientation effects. I was under the impression that a way of testing for this is to re-pack the sample, re-run and compare the spectra, as the level of preferred orientation should change between packings and so the spectra should differ slightly. Despite re-packing and comparing spectra from up to 8 scans the spectra overlay perfectly (within noise). This suggests to me that the refinement residuals are probably not due to preferred orientation. I am using Topas to refine the data and despite the comment above, the residuals seem best fitted using a spherical harmonic-prefered orientation model. I have also tried using other parameters to fit the residuals such as the occupancy of some of the sites, using anisotropic displacement parameters or anisotropic broadening effects, but the SH-PO fits best and seems the most physically reasonable treatment. As a test, I have also run the sample in capillary transmission geometry on another instrument, and the refined value of the SH-PO parameter is reduced. This seems to confirm that PO is the problem, but the question I have is how effective the re-packing comparison method is for confirming PO? If I see no difference in the spectra does it confirm that PO is NOT the problem or can I still have PO effects that are very repeatable upon repeat packing? If PO is not the problem in this case, are there any other structural or instrument effects that could lead to refinement residuals that would be well treated with a SH-PO model? Thank you all very much for any information or light you can shed on the subject. Ross Colman -- -- Daniel Chateigner Professeur, Université de Caen Basse-Normandie Co-editor Journal of Applied Crystallography, www.iucr.org Editor-in-Chief Texture, Stress and Microstructure, www.hindawi.com -- address: CRISMAT-ENSICAEN and IUT-Caen, Université de Caen Basse-Normandie, campus 2 6, Bd. M. Juin 14050 Caen, France tel: 33 (0)2 31 45 26 11 fax: 33 (0)2 31 95 16 00 daniel.chateig...@ensicaen.fr http://www.ecole.ensicaen.fr/~chateign/danielc/ -- A Quantitative Texture Analysis Course: http://qta.ecole.ensicaen.fr/ An Open Source for Crystallographic Data: the COD http://sdpd.univ-lemans.fr/cod/ --
RE: Preferred orientation?
This one's got my vote! Right little hornet's nest this one opened up. Goes to prove yet again that email is an imperfect way to communicate. Face-to-face discussions over a beer are much more preferable. Anyway I have to pay for enjoying a long weekend by fixing one diffractometer and doing the maintenance on two others so no time today for any jousting by email. Pam From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: May 18, 2008 7:44 PM To: Whitfield, Pamela; [EMAIL PROTECTED] Cc: rietveld_l@ill.fr Subject: RE: Preferred orientation? From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] The texture versus preferred orientation difference has some signficant blurry edges from a practical point of view. If you want it, it's texture. If you don't want it, it's preferred orientation! Matthew Matthew Rowles CSIRO Minerals Box 312 Clayton South, Victoria AUSTRALIA 3169 Ph: +61 3 9545 8892 Fax: +61 3 9562 8919 (site) Email: [EMAIL PROTECTED]
RE: Preferred orientation?
If I'm ever in the neighbourhood I'll be sure to pop along - although I'm actually a cider drinker (how's that for a confession in North America?) Anyone know a trick how to get the screws out of a D8 tube housing on a 3DOF without stripping the Torx threads other than getting a bigger hammer? - only joking BTW! These ones seem to be seriously tight. Waiting to hear from Madison on this one. The Scintag we have around is an old XDS2000 which isn't very well but it has other problems. Pam -Original Message- From: May, Frank [mailto:[EMAIL PROTECTED] Sent: May 20, 2008 9:38 AM To: Whitfield, Pamela Subject: RE: Preferred orientation? Well said, Pamela! Come to St. Louis and I'll buy you a Budweiser! By the way, do you know of any need for an old Scintag/Seifert diffractometer - just the theta and 2-theta box? It's vintage 1982 or so from a horizontal diffractometer. Frank May From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Tue 5/20/2008 8:30 AM To: [EMAIL PROTECTED]; [EMAIL PROTECTED] Cc: rietveld_l@ill.fr Subject: RE: Preferred orientation? This one's got my vote! Right little hornet's nest this one opened up. Goes to prove yet again that email is an imperfect way to communicate. Face-to-face discussions over a beer are much more preferable. Anyway I have to pay for enjoying a long weekend by fixing one diffractometer and doing the maintenance on two others so no time today for any jousting by email. Pam From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: May 18, 2008 7:44 PM To: Whitfield, Pamela; [EMAIL PROTECTED] Cc: rietveld_l@ill.fr Subject: RE: Preferred orientation? From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] The texture versus preferred orientation difference has some signficant blurry edges from a practical point of view. If you want it, it's texture. If you don't want it, it's preferred orientation! Matthew Matthew Rowles CSIRO Minerals Box 312 Clayton South, Victoria AUSTRALIA 3169 Ph: +61 3 9545 8892 Fax: +61 3 9562 8919 (site) Email: [EMAIL PROTECTED]
Re: Preferred orientation?
From a theoretical point of view, preferred orientation means orientational texture. There may be size textures, strain textures, grain boundary textures, dislocation textures etc. as well, although seldom in practical use. Therefrom, in practical use, texture is meant orientational texture and therefrom preferred orientation. Thank you very much, but I doubt the 'practical use' of limitations producing a God head ache as it delivers the 'discovery of the century'. What is the time continuum in the Q-space ? R u sure we r in time with the 'black box' and 'magic wand' before sun will kill the planet where some people just a few hours late. These ones seem to be seriously tight. Just give me back my hand I lost an another side. Face-to-face discussions over a beer are much more preferable. See you in Osaka, perhaps.
RE: Preferred orientation?
From a theoretical point of view, preferred orientation means orientational texture. There may be size textures, strain textures, grain boundary textures, dislocation textures etc. as well, although seldom in practical use. Therefrom, in practical use, texture is meant orientational texture and therefrom preferred orientation. Joerg Bergmann On Mon, 2008-05-19 at 07:26 +0200, Alan Hewat wrote: If you want it, it's texture. If you don't want it, it's preferred orientation! Preferred orientation is what you get with X-rays; neutrons are good for measuring texture :-) Seriously, texture is a term best applied to solid materials, where crystallites have preferred orientation due to the way the solid was formed or worked; texture can also be associated with strain broadening. Preferred orientation can also occur due to the way crystallites in powdered materials stack together because of their morphology (eg plates or needles), and is particularly difficult with small samples and particular sample geometries. Preferred orientation can also be associated with granularity (necessarily poor powder average). Interesting to see which topics generate the most heat, if not light (which are similarly related :-) Alan. __ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE [EMAIL PROTECTED] +33.476.98.41.68 http://www.NeutronOptics.com/hewat __
Re: Preferred orientation?
Dear all, If, I can to obtain the data from Bragg-Brentano geometry, obtained by very slowly steps (maybe to 20-30 seconds), Can I make a microstructure analysis (crystalline sizes and residual tensions) using Rietveld refinament? Best regards, Mario Macias UIS, Colombia --- Sorry for the late answer, I was out of office for a while. As Luca wrote, a complete texture analysis from a single Brag-Brentano scan is impossible. The PO correction as mentioned below is only seeing one sample direction, perpendicular to the sample surface. This is meaningful for powder samples packed in a holder having a PO prallel to the diffraction vector, but not for any other type of samples. I don't know anything about Fullprof, sorry. With best regards Reinhard Kleeberg [EMAIL PROTECTED] schrieb: Dear Reinhard, If I want make a microstructure analysis of data from Bragg-Brentano scan. Can I use spherical harmonic model and Rietveld refinament content into Fullprof? Best regars, Mario Macias UIS, Colombia. Luca, I understood Gerard's problem to have a measurement of a powder (of unknown particle shape) in Bragg-Brentano geometry, for structure refinement. As you said the graininess problem can be minimized primarily by grinding and to a certain extent by rotation and enhancing the divergence, axial as well as equatorial. It is obvious that overmuch graininess would prohibit any reasonable structure refinement or quantitative phase analysis, with or without the application of a harmonic model for intensity correction. Regarding the PO of powder mounts produced from minerals (big crystals) by grinding (McCrone mill, particle size 1-5 µm) and front- back- or sideloading the powder we must state that it is frequently present, of course strongly dependent from the particle shape (by cleavage) of the crystals. Not only platy or needle-like crystals tend to PO, but also cubes (e.g. halite), octahedra (e.g. fluorite), rhombohedra (e.g. calcite), pinakoids (e.g. feldspars) show PO because such crystals tend to stacking and orientation face by face, like bricks, in a real powder mount. The variation of the preparation technique (say sideloading instead of frontloading) does induce changes in PO what can be used to identify PO, as I understood Frank's recommendation. This is indeed the trick we apply every second week if a sample is suspicious to be not adequately prepared. I remember also that an intentionally enhancement of PO by preparation may also be applied for indexing and to add information in structure refinement (of multiple patterns, McCusker..). Of course the degree of orientation in such prepared powder samples is probably not as high as in your real textured samples, but m.r.d.'s in the magnitude of 3 (or down to 0.5) may appear. I believe this is weak texture in the context of texture analysis? For such (in my opinion also real and textured) powder samples, a model similar to the harmonics works fine for PO correction, provided that enough measurable peaks are in the pattern. E.g. the application of a model having 3 parameters to a cubic material showing only 2 different lattice planes in the one-dimensional scan must fail of course. Other problems may arise when two directions (e.g. h00 and 0k0) show strongly overlapping peaks by pseudo-symmetry, or when the intensity of a lot of hkl is very week for structural reasons. In all these cases the one-dimensional powder pattern contains not enough information to apply such a harmonic model, the model must be deactivated, a better (more random) sample must be prepared, the number of peaks must be enhanced... Thus, any PO correction in Rietveld refinement is second-best solution only. But in the daily routine work in Rietveld phase analysis, we can see that in the most cases the calculated PO correction factors are reasonable (reflecting the shape of the crystals, biggest faces get highest, directions inbetween lowest...), and the quantitative results are acceptable for such realisticly oriented powder samples what can produced by standard techniques. I believe this is an argument to apply these models, even though they are perhaps not able to decribe stronger PO or maybe unable to do a texture analysis (what is anyway impossible from a single Bragg-Brentano scan). Greetings Reinhard Luca Lutterotti schrieb: Reinhard, I stick with what Gerard said: But i have no other information that supports the existence of preferred orientation so what information give you the confirmation it is the powder mount responsible of preferred orientation. I work almost exclusively with image plate detectors and I can assure you that the graininess problem is appearing more often than the preferred orientation case. I am working on texture mostly so I am happy when you find them, but this case is not s frequent as people think and for sure not s frequent as graininess. I wait also confirmation from
RE: Preferred orientation?
_ From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] The texture versus preferred orientation difference has some signficant blurry edges from a practical point of view. If you want it, it's texture. If you don't want it, it's preferred orientation! Matthew Matthew Rowles CSIRO Minerals Box 312 Clayton South, Victoria AUSTRALIA 3169 Ph: +61 3 9545 8892 Fax: +61 3 9562 8919 (site) Email: [EMAIL PROTECTED]
Re: Preferred orientation?
The texture versus preferred orientation difference has some signficant blurry edges from a practical point of view. If you want it, it's texture. If you don't want it, it's preferred orientation! The discussion seems not for a nervous. A naive question of a baby watching adults in the mailing list: Is the 'preferred orientation' for crystallites, but is the 'texture' is for a whole sample where the crystallites have 'preferred orientation' ? I just want to know the theory how does one usually operate with this terms.
RE: Preferred orientation?
If you want it, it's texture. If you don't want it, it's preferred orientation! Preferred orientation is what you get with X-rays; neutrons are good for measuring texture :-) Seriously, texture is a term best applied to solid materials, where crystallites have preferred orientation due to the way the solid was formed or worked; texture can also be associated with strain broadening. Preferred orientation can also occur due to the way crystallites in powdered materials stack together because of their morphology (eg plates or needles), and is particularly difficult with small samples and particular sample geometries. Preferred orientation can also be associated with granularity (necessarily poor powder average). Interesting to see which topics generate the most heat, if not light (which are similarly related :-) Alan. __ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE [EMAIL PROTECTED] +33.476.98.41.68 http://www.NeutronOptics.com/hewat __
RE: Preferred orientation?
In fact I think you might find it helps quite a bit. Have a look at: http://img.chem.ucl.ac.uk/www/vickers/po/po.htm Martin Subject: RE: Preferred orientation?Date: Thu, 8 May 2008 10:55:12 -0400From: [EMAIL PROTECTED]: rietveld_l@ill.fr I do that myself but it doesn’t always help much if you’ve got something like wollastonite! J From: Martin [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 10:51 AMTo: [EMAIL PROTECTED]: RE: Preferred orientation? Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers Get fish-slapping on Messenger! Play Now _ http://clk.atdmt.com/UKM/go/msnnkmgl001009ukm/direct/01/
RE: Preferred orientation?
Any data from anywhere will lead the gullible and unwary up a certain creek without a particular implement and flat-plate is better than transmission for doing so. Older saying still (ca. 1503-07), thar be Dragons.Martin PS For anyone interested in an explanation of texture vs. PO, see: http://pd.chem.ucl.ac.uk/pdnn/inst1/texture1.htmhttp://pd.chem.ucl.ac.uk/pdnn/inst1/texture2.htm PPS for anyone interested in the TV ad that came to mind during this discussion, see: http://www.nationalarchives.gov.uk/films/1964to1979/filmpage_lonely.htmMartin Subject: RE: Preferred orientation?Date: Fri, 16 May 2008 13:05:24 -0400From: [EMAIL PROTECTED]: [EMAIL PROTECTED]: rietveld_l@ill.fr I'm not really arguing with you here as I prefer capillary data myself as it gives better data in many circumstances - however it can sometimes lead you up the garden path (another old saying!). I suppose what it boils down to is that needles are a pain as they can orientate whatever you do them (reflection or transmission). The texture versus preferred orientation difference has some signficant blurry edges from a practical point of view. Anyway - I'm on holiday so I'm going to put my brain to sleep and go and do some gardening! Pam From: Martin [mailto:[EMAIL PROTECTED]: Fri 16/05/2008 11:47 AMTo: Whitfield, Pamela; [EMAIL PROTECTED]: RE: Preferred orientation? Hi, happy RietveldersSome elements of confusion creeping in here. I think you said, Pam that transmission wont help much if it's wollastonite and what I'm saying is that it does and gave a pointer to a study that shows it. Indeed I don't claim transmission gets rid of PO either, but it does reduce it hugely which, if one reads back, is my claim here. The PO function in this case is merely to illustrate the point: 0.9 and a refined model vs. 1.6 and a bad refinement. The merits of various PO functions aren't important when it's the data that really count (or the counts that count, if you like). Why start out with bad data in the first place? As my old dad says, you can't make a silk purse out of a pigs ear. regards, Martin PS Just to clear up another possible point of confusion: large particles lead to texture effects, not preferred orientation. Subject: FW: Preferred orientation?Date: Fri, 16 May 2008 10:43:21 -0400From: [EMAIL PROTECTED]: rietveld_l@ill.fr I don't remember saying that reflection would work well with wollastonite only that a capillary won't get rid of the orientation (or at least that was my intention). As always this is going to vary from sample to sample, i.e. how large the particles are, aspect ratio of the particles, the diameter of the capillary, pure or mixture, etc. Orientation with needles is going to be more of a problem in a 0.3mm versus a 0.8mm capillary. Particle statistics are also a potential issue with samples like this. If they are large enough to orientate badly then the crystallites are probably large and the capillary will do a better job with the statistics The MD correction doesn't work very well for alot of these systems, SH is better as long as the correlations don't get out of hand (which they can quite easily). However 0.9 is still significantly orientated so it doesn't get rid of it by any means. With platey particles you can pretty much eliminate the preferential orientation with a capillary versus flat plate with significant impact of quantitative analysis results (paper published in Powder Diffraction a couple of years ago). I do have reflection data from 400 mesh wollastonite (albeit with MoKa from a high pressure gas cell) which can be fitted quite nicely with SH (lousy with MD). The quantitative analysis results from the carbonation are good enough to extract a rate constant which suits me nicely. The additional penetration of the MoKa should help with the stats in this case even if transparency becomes a problem - you can't win eh? Unfortunately it's quite difficult to completely decouple orientation, statistcs and microabsorption as they are all related to size. Pam From: Martin [mailto:[EMAIL PROTECTED]: Fri 16/05/2008 10:04 AMTo: Whitfield, Pamela; [EMAIL PROTECTED]: RE: Preferred orientation? In fact I think you might find it helps quite a bit. Have a look at: http://img.chem.ucl.ac.uk/www/vickers/po/po.htm Martin Subject: RE: Preferred orientation?Date: Thu, 8 May 2008 10:55:12 -0400From: [EMAIL PROTECTED]: rietveld_l@ill.fr I do that myself but it doesn’t always help much if you’ve got something like wollastonite! J From: Martin [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 10:51 AMTo: [EMAIL PROTECTED]: RE: Preferred orientation? Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers Get fish-slapping on Messenger! Play Now Get fish-slapping on Messenger! Play Now Get fish-slapping on Messenger! Play Now
Re: Preferred orientation?
Martin wrote: PS For anyone interested in an explanation of texture vs. PO, see: http://pd.chem.ucl.ac.uk/pdnn/inst1/texture1.htm http://pd.chem.ucl.ac.uk/pdnn/inst1/texture2.htm I disagree! The web page has confused texture with granularity. See: http://en.wikipedia.org/wiki/Texture_(crystalline) If you need a way to separate students in an exam, you could pretend to believe that texture refers to solid samples, but preferred orientation is for free flowing powders. eg: texture is a static property of some object, preferred orientation is what you got once with a particular sample holder. All the best, Jon
Re: Preferred orientation?
Luca, I understood Gerard's problem to have a measurement of a powder (of unknown particle shape) in Bragg-Brentano geometry, for structure refinement. As you said the graininess problem can be minimized primarily by grinding and to a certain extent by rotation and enhancing the divergence, axial as well as equatorial. It is obvious that overmuch graininess would prohibit any reasonable structure refinement or quantitative phase analysis, with or without the application of a harmonic model for intensity correction. Regarding the PO of powder mounts produced from minerals (big crystals) by grinding (McCrone mill, particle size 1-5 µm) and front- back- or sideloading the powder we must state that it is frequently present, of course strongly dependent from the particle shape (by cleavage) of the crystals. Not only platy or needle-like crystals tend to PO, but also cubes (e.g. halite), octahedra (e.g. fluorite), rhombohedra (e.g. calcite), pinakoids (e.g. feldspars) show PO because such crystals tend to stacking and orientation face by face, like bricks, in a real powder mount. The variation of the preparation technique (say sideloading instead of frontloading) does induce changes in PO what can be used to identify PO, as I understood Frank's recommendation. This is indeed the trick we apply every second week if a sample is suspicious to be not adequately prepared. I remember also that an intentionally enhancement of PO by preparation may also be applied for indexing and to add information in structure refinement (of multiple patterns, McCusker..). Of course the degree of orientation in such prepared powder samples is probably not as high as in your real textured samples, but m.r.d.'s in the magnitude of 3 (or down to 0.5) may appear. I believe this is weak texture in the context of texture analysis? For such (in my opinion also real and textured) powder samples, a model similar to the harmonics works fine for PO correction, provided that enough measurable peaks are in the pattern. E.g. the application of a model having 3 parameters to a cubic material showing only 2 different lattice planes in the one-dimensional scan must fail of course. Other problems may arise when two directions (e.g. h00 and 0k0) show strongly overlapping peaks by pseudo-symmetry, or when the intensity of a lot of hkl is very week for structural reasons. In all these cases the one-dimensional powder pattern contains not enough information to apply such a harmonic model, the model must be deactivated, a better (more random) sample must be prepared, the number of peaks must be enhanced... Thus, any PO correction in Rietveld refinement is second-best solution only. But in the daily routine work in Rietveld phase analysis, we can see that in the most cases the calculated PO correction factors are reasonable (reflecting the shape of the crystals, biggest faces get highest, directions inbetween lowest...), and the quantitative results are acceptable for such realisticly oriented powder samples what can produced by standard techniques. I believe this is an argument to apply these models, even though they are perhaps not able to decribe stronger PO or maybe unable to do a texture analysis (what is anyway impossible from a single Bragg-Brentano scan). Greetings Reinhard Luca Lutterotti schrieb: Reinhard, I stick with what Gerard said: But i have no other information that supports the existence of preferred orientation so what information give you the confirmation it is the powder mount responsible of preferred orientation. I work almost exclusively with image plate detectors and I can assure you that the graininess problem is appearing more often than the preferred orientation case. I am working on texture mostly so I am happy when you find them, but this case is not s frequent as people think and for sure not s frequent as graininess. I wait also confirmation from Gerard that his sample is a powder and it has plate like or fiber like particles. Otherwise I will investigate the graininess case that with a proper grinding or a spinner is easily resolvable. Also, for who think that because the harmonic model can fit it is for sure preferred orientation. I can just suggest to work for a while with real textured sample and the harmonic and see if there is really this relationship, you may be surprise by the result. cheers, Luca On May 8, 2008, at 3:08 PM, Reinhard Kleeberg wrote: Luca, speaking about powder samples, Frank is right. The PO of powder mounts is seldom reproducible and the filling technique is responsible for particle orientation, depending on particle shape, filling direction, pressure... In practice it is a nice trick to repeat the filling of the powder holder with different filling techniques to look for PO. Of course, sample graininess may be also a reason for not reproducible intensity, but these effects (rocks in the dust)
Re: Preferred orientation?
Dear Reinhard, If I want make a microstructure analysis of data from Bragg-Brentano scan. Can I use spherical harmonic model and Rietveld refinament content into Fullprof? Best regars, Mario Macias UIS, Colombia. Luca, I understood Gerard's problem to have a measurement of a powder (of unknown particle shape) in Bragg-Brentano geometry, for structure refinement. As you said the graininess problem can be minimized primarily by grinding and to a certain extent by rotation and enhancing the divergence, axial as well as equatorial. It is obvious that overmuch graininess would prohibit any reasonable structure refinement or quantitative phase analysis, with or without the application of a harmonic model for intensity correction. Regarding the PO of powder mounts produced from minerals (big crystals) by grinding (McCrone mill, particle size 1-5 µm) and front- back- or sideloading the powder we must state that it is frequently present, of course strongly dependent from the particle shape (by cleavage) of the crystals. Not only platy or needle-like crystals tend to PO, but also cubes (e.g. halite), octahedra (e.g. fluorite), rhombohedra (e.g. calcite), pinakoids (e.g. feldspars) show PO because such crystals tend to stacking and orientation face by face, like bricks, in a real powder mount. The variation of the preparation technique (say sideloading instead of frontloading) does induce changes in PO what can be used to identify PO, as I understood Frank's recommendation. This is indeed the trick we apply every second week if a sample is suspicious to be not adequately prepared. I remember also that an intentionally enhancement of PO by preparation may also be applied for indexing and to add information in structure refinement (of multiple patterns, McCusker..). Of course the degree of orientation in such prepared powder samples is probably not as high as in your real textured samples, but m.r.d.'s in the magnitude of 3 (or down to 0.5) may appear. I believe this is weak texture in the context of texture analysis? For such (in my opinion also real and textured) powder samples, a model similar to the harmonics works fine for PO correction, provided that enough measurable peaks are in the pattern. E.g. the application of a model having 3 parameters to a cubic material showing only 2 different lattice planes in the one-dimensional scan must fail of course. Other problems may arise when two directions (e.g. h00 and 0k0) show strongly overlapping peaks by pseudo-symmetry, or when the intensity of a lot of hkl is very week for structural reasons. In all these cases the one-dimensional powder pattern contains not enough information to apply such a harmonic model, the model must be deactivated, a better (more random) sample must be prepared, the number of peaks must be enhanced... Thus, any PO correction in Rietveld refinement is second-best solution only. But in the daily routine work in Rietveld phase analysis, we can see that in the most cases the calculated PO correction factors are reasonable (reflecting the shape of the crystals, biggest faces get highest, directions inbetween lowest...), and the quantitative results are acceptable for such realisticly oriented powder samples what can produced by standard techniques. I believe this is an argument to apply these models, even though they are perhaps not able to decribe stronger PO or maybe unable to do a texture analysis (what is anyway impossible from a single Bragg-Brentano scan). Greetings Reinhard Luca Lutterotti schrieb: Reinhard, I stick with what Gerard said: But i have no other information that supports the existence of preferred orientation so what information give you the confirmation it is the powder mount responsible of preferred orientation. I work almost exclusively with image plate detectors and I can assure you that the graininess problem is appearing more often than the preferred orientation case. I am working on texture mostly so I am happy when you find them, but this case is not s frequent as people think and for sure not s frequent as graininess. I wait also confirmation from Gerard that his sample is a powder and it has plate like or fiber like particles. Otherwise I will investigate the graininess case that with a proper grinding or a spinner is easily resolvable. Also, for who think that because the harmonic model can fit it is for sure preferred orientation. I can just suggest to work for a while with real textured sample and the harmonic and see if there is really this relationship, you may be surprise by the result. cheers, Luca On May 8, 2008, at 3:08 PM, Reinhard Kleeberg wrote: Luca, speaking about powder samples, Frank is right. The PO of powder mounts is seldom reproducible and the filling technique is responsible for particle orientation, depending on particle shape, filling direction, pressure... In practice it is a nice trick to
Re: Preferred orientation?
On May 8, 2008, at 12:30 AM, May, Frank wrote: You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. Not true, preferred orientation or texture are perfectly reproducible, provided you use the same sample orientation. What is not reproducible and probably what Frank May is referring to is not preferred orientation but graininess or few big grains that do not guarantee the correct statistic. So if you need to check for graininess, you just move a little your sample, so the beam covers a different area on the sample. If you think you have texturte, to check for it you have to change the sample orientation to see a change. Beware that in a Bragg-Brentano instrument turning around the axis normal to the sample surface is not a valid change in orientation as nothing will change for texture; you have to change the sample inclination instead (omega or chi). Best Regards, Luca Lutterotti That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20-degrees 2- Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa does not seem to be the cause. Atomic positions also look sensible. And again, data at high angle looks ok. Is the scattering angle dependence of the atomic positions the same as for the thermal parameters? (I cannot remember that, but i am pretty sure it is not). Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other factor that might cause this problem is preferred orientation: I have tried to find a hkl dependence in the overestimated and underestimated peaks but i could not find any. If i try to model preferred orientation with spherical harmonics the problems disappears nicely. The problem is how to justify the existence of preferred orientation. The crystal system is orthorhombic. But i have no other information that supports the existence of preferred orientation. Is there any other problem that I cannot think of?Is the preferred orientation correction masking any of these other problems I cannot think of? Regards Gerard Heriot-Watt University is a Scottish charity registered under charity number SC000278.
Re: Preferred orientation?
Luca, speaking about powder samples, Frank is right. The PO of powder mounts is seldom reproducible and the filling technique is responsible for particle orientation, depending on particle shape, filling direction, pressure... In practice it is a nice trick to repeat the filling of the powder holder with different filling techniques to look for PO. Of course, sample graininess may be also a reason for not reproducible intensity, but these effects (rocks in the dust) ar mostly hard to correct successfully by spherical harmonics as Gerard stated for his problem. In any case, the problem sounds to be related to sample preparation. Reinhard Luca Lutterotti schrieb: On May 8, 2008, at 12:30 AM, May, Frank wrote: You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. Not true, preferred orientation or texture are perfectly reproducible, provided you use the same sample orientation. What is not reproducible and probably what Frank May is referring to is not preferred orientation but graininess or few big grains that do not guarantee the correct statistic. So if you need to check for graininess, you just move a little your sample, so the beam covers a different area on the sample. If you think you have texturte, to check for it you have to change the sample orientation to see a change. Beware that in a Bragg-Brentano instrument turning around the axis normal to the sample surface is not a valid change in orientation as nothing will change for texture; you have to change the sample inclination instead (omega or chi). Best Regards, Luca Lutterotti That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20-degrees 2- Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa does not seem to be the cause. Atomic positions also look sensible. And again, data at high angle looks ok. Is the scattering angle dependence of the atomic positions the same as for the thermal parameters? (I cannot remember that, but i am pretty sure it is not). Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other factor that might cause this problem is preferred orientation: I have tried to find a hkl dependence in the overestimated and underestimated peaks but i could not find any. If i try to model preferred orientation with spherical harmonics the problems disappears nicely. The problem is how to justify the existence of preferred orientation. The crystal system is orthorhombic. But i have no other information that supports the existence of preferred orientation. Is there any other problem that I cannot think of?Is the preferred orientation correction masking any of these other problems I cannot think of? Regards Gerard Heriot-Watt University is a Scottish charity registered under charity number SC000278.
Re: Preferred orientation?
Another way to check or convince yourself of preferred orientation is to take note what is happening when grinding the sample. Some crystallite samples will fracture along a particular plane upon grinding and this summed up for the whole sample may produce preferred orientation. Alternatively, by not grinding up the sample, you may not achieve a uniform crystallite size, many crystals grow along one or two preferred axes and grinding will help in this case. To convince yourself the crystallite morphology is giving rise to preferred orientation, begin by grinding your sample to oblivion (not so much to make it appear amorphous), you will have broaden peaks, and expected intense peaks should reduce in intensity relative to other peaks. Then you work up from there, decreasing the amount of grinding until no grinding and you should see the relative peak intensity increase (or decrease if the case may be) more for some peaks than you would expect from differing crystallite sizes. Then from the having determined the correct degree of grinding (to ensure a good FWHM) the preferred orientation correction can be put in place with greater confidence. An aside - mismatches in crystallite sizes or small crystallites will result in peak broadening, though when the crystallite size approaches single crystal size (using small amount of sample), then coherent interference will breakdown for the sample as a whole and on a powder diffractometer this will be identified with a few incredibly large intense peaks. The technique of mounting samples will also reduce or increase preferred orientation. Back filling is a good skill to learn, it creates a more random distribution of crystallite orientation. Acetone slurries on the other hand will enhance the ability of the crystallites to layer themselves in a preferred direction. Regards William On May 8, 2008, at 12:30 AM, May, Frank wrote: You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. Not true, preferred orientation or texture are perfectly reproducible, provided you use the same sample orientation. What is not reproducible and probably what Frank May is referring to is not preferred orientation but graininess or few big grains that do not guarantee the correct statistic. So if you need to check for graininess, you just move a little your sample, so the beam covers a different area on the sample. If you think you have texturte, to check for it you have to change the sample orientation to see a change. Beware that in a Bragg-Brentano instrument turning around the axis normal to the sample surface is not a valid change in orientation as nothing will change for texture; you have to change the sample inclination instead (omega or chi). Best Regards, Luca Lutterotti That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20-degrees 2- Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa does not seem to be the cause. Atomic positions also look sensible. And again, data at high angle looks ok. Is the scattering angle dependence of the atomic positions the same as for the thermal parameters? (I cannot remember that, but i am pretty sure it is not). Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other factor that might cause this problem is preferred orientation: I have tried to find a hkl dependence in the overestimated and underestimated peaks but i could not find any. If i try to model preferred orientation with spherical harmonics the problems disappears nicely. The problem is how to justify the existence of preferred orientation. The crystal system is orthorhombic. But i have no other information that supports the existence of preferred orientation. Is there any other
RE: Preferred orientation?
If you have one, stick the sample on a GADDS system or similar with a 2D detector and a large collimator. You'll find out if your sample is grainy (or rocks in dust as Reinhard put it) in less than a minute. A nicely micronized sample should give nice even rings with a 1mm collimator. Pam -Original Message- From: William Bisson [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 9:29 AM To: rietveld_l@ill.fr Subject: Re: Preferred orientation? Another way to check or convince yourself of preferred orientation is to take note what is happening when grinding the sample. Some crystallite samples will fracture along a particular plane upon grinding and this summed up for the whole sample may produce preferred orientation. Alternatively, by not grinding up the sample, you may not achieve a uniform crystallite size, many crystals grow along one or two preferred axes and grinding will help in this case. To convince yourself the crystallite morphology is giving rise to preferred orientation, begin by grinding your sample to oblivion (not so much to make it appear amorphous), you will have broaden peaks, and expected intense peaks should reduce in intensity relative to other peaks. Then you work up from there, decreasing the amount of grinding until no grinding and you should see the relative peak intensity increase (or decrease if the case may be) more for some peaks than you would expect from differing crystallite sizes. Then from the having determined the correct degree of grinding (to ensure a good FWHM) the preferred orientation correction can be put in place with greater confidence. An aside - mismatches in crystallite sizes or small crystallites will result in peak broadening, though when the crystallite size approaches single crystal size (using small amount of sample), then coherent interference will breakdown for the sample as a whole and on a powder diffractometer this will be identified with a few incredibly large intense peaks. The technique of mounting samples will also reduce or increase preferred orientation. Back filling is a good skill to learn, it creates a more random distribution of crystallite orientation. Acetone slurries on the other hand will enhance the ability of the crystallites to layer themselves in a preferred direction. Regards William On May 8, 2008, at 12:30 AM, May, Frank wrote: You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. Not true, preferred orientation or texture are perfectly reproducible, provided you use the same sample orientation. What is not reproducible and probably what Frank May is referring to is not preferred orientation but graininess or few big grains that do not guarantee the correct statistic. So if you need to check for graininess, you just move a little your sample, so the beam covers a different area on the sample. If you think you have texturte, to check for it you have to change the sample orientation to see a change. Beware that in a Bragg-Brentano instrument turning around the axis normal to the sample surface is not a valid change in orientation as nothing will change for texture; you have to change the sample inclination instead (omega or chi). Best Regards, Luca Lutterotti That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20-degrees 2- Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa does not seem to be the cause. Atomic positions also look sensible. And again, data at high angle looks ok. Is the scattering angle dependence of the atomic positions the same as for the thermal parameters? (I cannot remember that, but i am pretty sure it is not). Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other factor that might cause
RE: Preferred orientation?
Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers _ Be a Hero and Win with Iron Man http://clk.atdmt.com/UKM/go/msnnkmgl001009ukm/direct/01/
RE: Preferred orientation?
I do that myself but it doesn't always help much if you've got something like wollastonite! J From: Martin [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 10:51 AM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers Get fish-slapping on Messenger! Play Now http://clk.atdmt.com/UKM/go/msnnkmgl001008ukm/direct/01/
RE: Preferred orientation?
Hi all, This thread gives me a chance to ask a question I've had for a long time. I've heard about these large chambers where you can mix your sample with a binder and have it fall out as small powder spheres to avoid preferred orientation in Bragg-Brentano geometry. But, my samples are mostly between 10 and 50 milligrams in size. Does anyone know a way to mount them without preferred orientation? Thank you! - Kurt From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 9:16 AM To: Kurt Leinenweber Subject: RE: Preferred orientation? It's one of the classic needle-shaped materials - it gives lovely SEM images if you can avoid charging From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 12:12 PM To: Whitfield, Pamela Subject: RE: Preferred orientation? HI Pamela/all, This sounds intriguing.. why is wollastonite a problem in capillary transmission? Is it needle-like? - Kurt From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 7:55 AM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? I do that myself but it doesn't always help much if you've got something like wollastonite! :-) From: Martin [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 10:51 AM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers Get fish-slapping on Messenger! Play Now http://clk.atdmt.com/UKM/go/msnnkmgl001008ukm/direct/01/
RE: Preferred orientation?
Kurt, An old way used for alloys is: grease the surface of the sample holder (preferably backgroundless) with some sticky stuff and sieve the powder onto it. The particles will fall down and stuck at random orientations, unless they are large plates or needles. Peter Zavalij Director, X-ray Crystallographic Center Department of Chemistry and Biochemistry 091 Chemistry Building University of Maryland College Park, MD 20742-4454 Phone: (301)405-1861 Lab: (301)405-1861 Fax: (301)314-9121 E-mail: [EMAIL PROTECTED] http://www.chem.umd.edu/facility/xray/ _ From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 12:37 PM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? Hi all, This thread gives me a chance to ask a question I've had for a long time. I've heard about these large chambers where you can mix your sample with a binder and have it fall out as small powder spheres to avoid preferred orientation in Bragg-Brentano geometry. But, my samples are mostly between 10 and 50 milligrams in size. Does anyone know a way to mount them without preferred orientation? Thank you! - Kurt _ From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 9:16 AM To: Kurt Leinenweber Subject: RE: Preferred orientation? It's one of the classic needle-shaped materials - it gives lovely SEM images if you can avoid charging From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 12:12 PM To: Whitfield, Pamela Subject: RE: Preferred orientation? HI Pamela/all, This sounds intriguing.. why is wollastonite a problem in capillary transmission? Is it needle-like? - Kurt _ From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 7:55 AM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? I do that myself but it doesn't always help much if you've got something like wollastonite! :-) From: Martin [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 10:51 AM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers _ Get fish-slapping on Messenger! Play Now http://clk.atdmt.com/UKM/go/msnnkmgl001008ukm/direct/01/
Re: Preferred orientation?
Reinhard, I stick with what Gerard said: But i have no other information that supports the existence of preferred orientation so what information give you the confirmation it is the powder mount responsible of preferred orientation. I work almost exclusively with image plate detectors and I can assure you that the graininess problem is appearing more often than the preferred orientation case. I am working on texture mostly so I am happy when you find them, but this case is not s frequent as people think and for sure not s frequent as graininess. I wait also confirmation from Gerard that his sample is a powder and it has plate like or fiber like particles. Otherwise I will investigate the graininess case that with a proper grinding or a spinner is easily resolvable. Also, for who think that because the harmonic model can fit it is for sure preferred orientation. I can just suggest to work for a while with real textured sample and the harmonic and see if there is really this relationship, you may be surprise by the result. cheers, Luca On May 8, 2008, at 3:08 PM, Reinhard Kleeberg wrote: Luca, speaking about powder samples, Frank is right. The PO of powder mounts is seldom reproducible and the filling technique is responsible for particle orientation, depending on particle shape, filling direction, pressure... In practice it is a nice trick to repeat the filling of the powder holder with different filling techniques to look for PO. Of course, sample graininess may be also a reason for not reproducible intensity, but these effects (rocks in the dust) ar mostly hard to correct successfully by spherical harmonics as Gerard stated for his problem. In any case, the problem sounds to be related to sample preparation. Reinhard Luca Lutterotti schrieb: On May 8, 2008, at 12:30 AM, May, Frank wrote: You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. Not true, preferred orientation or texture are perfectly reproducible, provided you use the same sample orientation. What is not reproducible and probably what Frank May is referring to is not preferred orientation but graininess or few big grains that do not guarantee the correct statistic. So if you need to check for graininess, you just move a little your sample, so the beam covers a different area on the sample. If you think you have texturte, to check for it you have to change the sample orientation to see a change. Beware that in a Bragg-Brentano instrument turning around the axis normal to the sample surface is not a valid change in orientation as nothing will change for texture; you have to change the sample inclination instead (omega or chi). Best Regards, Luca Lutterotti That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20- degrees 2- Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa does not seem to be the cause. Atomic positions also look sensible. And again, data at high angle looks ok. Is the scattering angle dependence of the atomic positions the same as for the thermal parameters? (I cannot remember that, but i am pretty sure it is not). Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other factor that might cause this problem is preferred orientation: I have tried to find a hkl dependence in the overestimated and underestimated peaks but i could not find any. If i try to model preferred orientation with spherical harmonics the problems disappears nicely. The problem is how to justify the existence of preferred orientation. The crystal system is orthorhombic. But i have no other information that supports the existence of preferred
RE: Preferred orientation?
Hairspray works quite well for thin samples (no comment from me as to whether environmentally friendly non-aerosol is as good as aerosol as I haven't tested it yet). Getting a nice even coverage from the sieve can take a bit of practice as well. Together with nail varnish for sealing capillaries visitors to my lab must sometimes think I'm very vain! Pam From: Peter Y. Zavalij [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 12:48 PM To: 'Kurt Leinenweber' Cc: rietveld_l@ill.fr Subject: RE: Preferred orientation? Kurt, An old way used for alloys is: grease the surface of the sample holder (preferably backgroundless) with some sticky stuff and sieve the powder onto it. The particles will fall down and stuck at random orientations, unless they are large plates or needles. Peter Zavalij Director, X-ray Crystallographic Center Department of Chemistry and Biochemistry 091 Chemistry Building University of Maryland College Park, MD 20742-4454 Phone: (301)405-1861 Lab: (301)405-1861 Fax: (301)314-9121 E-mail: [EMAIL PROTECTED] http://www.chem.umd.edu/facility/xray/ From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 12:37 PM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? Hi all, This thread gives me a chance to ask a question I've had for a long time. I've heard about these large chambers where you can mix your sample with a binder and have it fall out as small powder spheres to avoid preferred orientation in Bragg-Brentano geometry. But, my samples are mostly between 10 and 50 milligrams in size. Does anyone know a way to mount them without preferred orientation? Thank you! - Kurt From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 9:16 AM To: Kurt Leinenweber Subject: RE: Preferred orientation? It's one of the classic needle-shaped materials - it gives lovely SEM images if you can avoid charging From: Kurt Leinenweber [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 12:12 PM To: Whitfield, Pamela Subject: RE: Preferred orientation? HI Pamela/all, This sounds intriguing.. why is wollastonite a problem in capillary transmission? Is it needle-like? - Kurt From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Thursday, May 08, 2008 7:55 AM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? I do that myself but it doesn't always help much if you've got something like wollastonite! J From: Martin [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 10:51 AM To: rietveld_l@ill.fr Subject: RE: Preferred orientation? Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers Get fish-slapping on Messenger! Play Now http://clk.atdmt.com/UKM/go/msnnkmgl001008ukm/direct/01/
(Fwd) Re: Preferred orientation?
I very much agree with Luca in that graininess is not given the importance it actually has. Older textbooks like Klug-Alexander or Peiser considered graininess to some depth, and simple estimations show that in a usual BB sample, the number of grains in Bragg condition may be as low as 1 for a grain diameter of 10 Å! For Kurt: try Debye Scherrer (or better Gandolfi) if you can accept the peak broadening. best miguel --- Forwarded message follows --- Date sent: Thu, 08 May 2008 19:17:53 +0200 From: Luca Lutterotti [EMAIL PROTECTED] Subject:Re: Preferred orientation? To: [EMAIL PROTECTED] Copies to: rietveld_l@ill.fr [ Double-click this line for list subscription options ] Reinhard, I stick with what Gerard said: But i have no other information that supports the existence of preferred orientation so what information give you the confirmation it is the powder mount responsible of preferred orientation. I work almost exclusively with image plate detectors and I can assure you that the graininess problem is appearing more often than the preferred orientation case. I am working on texture mostly so I am happy when you find them, but this case is not s frequent as people think and for sure not s frequent as graininess. I wait also confirmation from Gerard that his sample is a powder and it has plate like or fiber like particles. Otherwise I will investigate the graininess case that with a proper grinding or a spinner is easily resolvable. Also, for who think that because the harmonic model can fit it is for sure preferred orientation. I can just suggest to work for a while with real textured sample and the harmonic and see if there is really this relationship, you may be surprise by the result. cheers, Luca On May 8, 2008, at 3:08 PM, Reinhard Kleeberg wrote: Luca, speaking about powder samples, Frank is right. The PO of powder mounts is seldom reproducible and the filling technique is responsible for particle orientation, depending on particle shape, filling direction, pressure... In practice it is a nice trick to repeat the filling of the powder holder with different filling techniques to look for PO. Of course, sample graininess may be also a reason for not reproducible intensity, but these effects (rocks in the dust) ar mostly hard to correct successfully by spherical harmonics as Gerard stated for his problem. In any case, the problem sounds to be related to sample preparation. Reinhard Luca Lutterotti schrieb: On May 8, 2008, at 12:30 AM, May, Frank wrote: You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. Not true, preferred orientation or texture are perfectly reproducible, provided you use the same sample orientation. What is not reproducible and probably what Frank May is referring to is not preferred orientation but graininess or few big grains that do not guarantee the correct statistic. So if you need to check for graininess, you just move a little your sample, so the beam covers a different area on the sample. If you think you have texturte, to check for it you have to change the sample orientation to see a change. Beware that in a Bragg-Brentano instrument turning around the axis normal to the sample surface is not a valid change in orientation as nothing will change for texture; you have to change the sample inclination instead (omega or chi). Best Regards, Luca Lutterotti That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20- degrees 2- Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa
Re: Preferred orientation?
Dear Gerard, The low-angle intensity problem might come from disordered regions, like water or solvent molecules occupying some voids. Also, poor modeling of weakly scattering atoms, like hydrogen, may lead to the similar problem. Best regards, Yaroslav http://filinchuk.com ===8==Original message text=== Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa does not seem to be the cause. Atomic positions also look sensible. And again, data at high angle looks ok. Is the scattering angle dependence of the atomic positions the same as for the thermal parameters? (I cannot remember that, but i am pretty sure it is not). Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other factor that might cause this problem is preferred orientation: I have tried to find a hkl dependence in the overestimated and underestimated peaks but i could not find any. If i try to model preferred orientation with spherical harmonics the problems disappears nicely. The problem is how to justify the existence of preferred orientation. The crystal system is orthorhombic. But i have no other information that supports the existence of preferred orientation. Is there any other problem that I cannot think of?Is the preferred orientation correction masking any of these other problems I cannot think of? Regards Gerard -- Heriot-Watt University is a Scottish charity registered under charity number SC000278. ===8===End of original message text===
RE: Preferred orientation?
You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20-degrees 2-Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa does not seem to be the cause. Atomic positions also look sensible. And again, data at high angle looks ok. Is the scattering angle dependence of the atomic positions the same as for the thermal parameters? (I cannot remember that, but i am pretty sure it is not). Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other factor that might cause this problem is preferred orientation: I have tried to find a hkl dependence in the overestimated and underestimated peaks but i could not find any. If i try to model preferred orientation with spherical harmonics the problems disappears nicely. The problem is how to justify the existence of preferred orientation. The crystal system is orthorhombic. But i have no other information that supports the existence of preferred orientation. Is there any other problem that I cannot think of?Is the preferred orientation correction masking any of these other problems I cannot think of? Regards Gerard Heriot-Watt University is a Scottish charity registered under charity number SC000278.
Re: Preferred orientation and clay minerals
At 12:04 19/01/99 +0100, Lubo wrote: Hi Armel, I am sure you will not be satisfied by this answer, but my solution to this problem is : USE transmission geometry. We have been doing it for years getting nice results for kaolinites, micas, vermiculites and smectites, not to speak of some organic plates. No need to add -osils (what's that, BTW). Lubo P.S. I am not adding the type of diffractometer, but if you wish I can scan a sample for you After some scans, and up to now, the technique of mixing the sample with a solid filler material (like Cab-osil M5) or dusting a sample through a sieve are the clear (less expensive) winners of this private round robin, provided the mixture is inserted in a side-loading sample holder and never packed. Moreover a mixture fifty-fifty in volume is recommended with the amorphous solid filler technique. Transmission geometry favours of course preferred orientation directions at 90° from the reflection geometry one. With platelet habit clay minerals and 00l orientation by reflection geometry (and say, a March-Dollase parameter which can be easily as low as 0.5 when a sample is packed), one obtains with transmission geometry the reverse : a March-Dollase parameter as high as 1.5, selecting the same 00l orientation direction by the Rietveld method (no orientation corresponding to MD=1.0). The problem of course is that Rietveld refinements are of quite low quality when the MD is so different from 1 Best, Armel Le Bail - Universite du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France http://www.cristal.org/
