Re: Problem of low-angle
Dear Chuisy, The broad bump you are experiencing at low angle may be to do with the slits. One has to be careful with these new XRD machines which have variable slits, to make sure the divergent and the anti-scattering slits are equal and if you choose to keep the slits fixed, which usually results in a broad peak, the footprint is small. However, this will result in poor high angle data. It does require a deal of playing around with the slits to see what each of the effects are. The main purpose of variable slits is to have consistant data throughout a range of 2Theta, which it will do, however, it does make refinement just that little harder, having to take into account changing slit width over 2Theta. If anyone has any good tips on this matter, I would be very interested to know. Regards William Bisson On Thu, 3 Jun 2004 [EMAIL PROTECTED] wrote: Dear all, We are recently installing a XRD, and there is a huge peak occurred in the 3-7deg (2T) which is masking the useful signals from the sample. How can we eliminate the big peak there? We use 40kW, 30mA CuKa x-ray, divergent and receving slits are used, many thanks, stephen Chui
Re: preferential orientation
Dear Christian (and others) your reply about preferred orientation probabaly not being a problem with a cubic mineral is of interest to me. This is something I have wondered about in the past because of observations I have made on common salt, so I thought I would contribute to this discussion in the hope that I can learn something. I am not a crystallographer so my thinking about this may be completely wrong, but if you pack a mineral like salt into a holder such that all the little cubes lined up (to some extent) in a regular way, would this show up as preferred orientation in an XRD pattern? or does it make no difference because the mineral is cubic? I pose my question because I have observed apparently different XRD patterns for salt when prepared in different ways. I guess my broader question is can cubic minerals show preferred orientation? I had convinced myself they can, your comments have now made me wonder about this again. sincerely, steve Dr Stephen Hillier Macaulay Institute Craigiebuckler ABERDEEN AB15 8QH UK Tel. +44 (0) 1224 318611 Fax. +44 (0) 1224 311556 e-mail: [EMAIL PROTECTED] http://www.macaulay.ac.uk
Re: Dislocation loop and broadening
In this case it is case it is expected that there will be formation of dislocation loops in the material due to irradiation. Well, simulations which would be made on the same material in the case of different kinds of dislocations and defaults could say if really the line-broadening is the finger-print of some defects when they are dominant. Do such simulations were made ? I mean simulation with large models, knowing precisely all the atom positions, including in the neighbouring of the defects, and using the Debye formula for estimating the interference function. I do not think that such credible simulations were made. Most simulations are done by using approximated formula giving directly some peak shapes at expected diffraction positions. Defects are statistically approximated. This has nothing relation with any reality of a large model where atoms have to accomodate the defects (interatomic distances have to be realistic). I am afraid that such credible simulations could reveal that very different kinds of defaults could give rise to almost the same broadening on the powder patterns. Because a powder pattern is a mess ;-). 3 dimensions gathered in one... Best wishes, Armel
Re: preferential orientation
Dear Stephen, You are quite correct. I was going to make the same reply to Christian's comment also using NaCl as an example. What I would add though is that NaCl may be more prone to preferred orientation as the cubes are close to perfect - in contrast to say cubic zeolites where I have observed more rounded corners (e.g. as one has on a pair of dice). Cubes with rounded corners probably orient more randomly, though there is still the possibility to line up with the flat face parallel to the surface when flattened with a glass micorscope slide during sample preparation. I also find that well ground NaCl is less prone to texture. Jeremy Karl Cockcroft. --- Dr Jeremy Karl CockcroftEmail: [EMAIL PROTECTED] School of Crystallography Tel: 020 7631 6849 (office) or 6853 (laboratory) Birkbeck CollegeTel: 020 7631 6803 (fax) or 6800 (secretary) Malet Street, London WC1E 7HX URL: http://img.cryst.bbk.ac.uk/www/img.htm ---
Re: preferential orientation
your reply about preferred orientation probabaly not being a problem with a cubic mineral is of interest to me. I can testify to have seen huge preferred orientation occuring in some cubic perovskites, at least. Too much intensity in those h00. Armel
Re: Problem of low-angle
Hi, Looking at the attached file, my opinion is that this effect is caused by a quite large divergent (or both divergent and anti-scattering) slits. It is not normal to have over 1000 counts at 5 degrees 2-theta. This could be due either by a portion of incident beam or by a scattering from the sample holder. It is not clear if your XRD is equipped with variable slits. If yes, William is correct - try to adjust them properly. I was working some years ago with variable slits diffractometer and as I remember I had hard time trying to collect good quality data below 3 degrees 2-theta. Now I have a fixed slits goniometer and I am easily getting pretty good patterns even below 1 degree 2-theta - by appropriate setting of the slits. I think this is one important benefit of using fixed slits - the higher quality of low theta scans. Dear Chuisy, with appropriate slits arrangement, at 5 degrees 2-theta you should have almost horizontal background level and every peak should be well displayed on the pattern. So, if you have fixed slit system - try with smaller divergent and anti-scattering slits. It would be better if you use as a test sample something that provides peaks below 5 degrees in order to check and control the quality of peaks appearance on the pattern while adjusting the slits. Don't forget: for good quality low theta data it is critical to have a large sample width (or length - depends on the view). My sample holders for low theta data collection are rectangular shaped 18x35 mm with different depths. The beam must be kept as much as it is possible within the sample area. Good luck, Srebri /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\ Dr. Sr. Petrov, PXRD Analyses, Dept. of Chemistry, University of Toronto Tel/Fax: (416)-978-1389. - Original Message - From: William Bisson [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Thursday, June 03, 2004 6:21 AM Dear Chuisy, The broad bump you are experiencing at low angle may be to do with the slits. One has to be careful with these new XRD machines which have variable slits, to make sure the divergent and the anti-scattering slits are equal and if you choose to keep the slits fixed, which usually results in a broad peak, the footprint is small. However, this will result in poor high angle data. It does require a deal of playing around with the slits to see what each of the effects are. The main purpose of variable slits is to have consistant data throughout a range of 2Theta, which it will do, however, it does make refinement just that little harder, having to take into account changing slit width over 2Theta. If anyone has any good tips on this matter, I would be very interested to know. Regards William Bisson On Thu, 3 Jun 2004 [EMAIL PROTECTED] wrote: Dear all, We are recently installing a XRD, and there is a huge peak occurred in the 3-7deg (2T) which is masking the useful signals from the sample. How can we eliminate the big peak there? We use 40kW, 30mA CuKa x-ray, divergent and receving slits are used, many thanks, stephen Chui
Re: Problem of low-angle
Dear William, Thank you for your kind suggestion. Well, our XRD is not a very new model, we use a fixed divergent slit, receving slit and antiscattering slits. But I need to check them whether the divergent and anti-scattering slits are in the same sizes. When I added the anti-scatter slit, the bump did reduce in size but still there. In this case, can you teach me what precautions we need to concern when the Ni-filter, beam mask, sample holder (glass plate) and the receiving slits are used? I will make some reading as well, however I do willing to hear some practical experience from XRD users. many thanks, stephen chui Quoting William Bisson [EMAIL PROTECTED]: Dear Chuisy, The broad bump you are experiencing at low angle may be to do with the slits. One has to be careful with these new XRD machines which have variable slits, to make sure the divergent and the anti-scattering slits are equal and if you choose to keep the slits fixed, which usually results in a broad peak, the footprint is small. However, this will result in poor high angle data. It does require a deal of playing around with the slits to see what each of the effects are. The main purpose of variable slits is to have consistant data throughout a range of 2Theta, which it will do, however, it does make refinement just that little harder, having to take into account changing slit width over 2Theta. If anyone has any good tips on this matter, I would be very interested to know. Regards William Bisson On Thu, 3 Jun 2004 [EMAIL PROTECTED] wrote: Dear all, We are recently installing a XRD, and there is a huge peak occurred in the 3-7deg (2T) which is masking the useful signals from the sample. How can we eliminate the big peak there? We use 40kW, 30mA CuKa x-ray, divergent and receving slits are used, many thanks, stephen Chui
[no subject]
Hi All, There is a new version of Windows GSAS on the CCP14 web server. The biggest change is that it will now do Pawley style extractions for one set of reflections from multiple powder patterns. This is done inside GENLES. See below for note on this other changes. Linux other versions will follow soon. As usual please let me know of bugs, etc. Bob Von Dreele GSAS News... June 3, 2004 1. Fixed errors in arcsin, arccos arctan2 routines that gave misbehavior in texture and possibly other calculations when the result angle was 0, +/-90 or 180. 2. Fixed a mishandling of the last profile point under some circumstances. 3. Made defaults for sample angles for sph. harmonics as follows: constant wavelength x-ray: omega=0, chi=0, phi=0; omega follows 1/2*2-theta for Bragg-Brentano neutron (TOF or CW) : omega=0, chi=90, phi=0; omega fixed for Debye-Scherrer These can be overridden, for example, in the iparm file by entering records similar to: INS 1CHI 90. INS 1OMEGA 0. INS 1PHI 0. for Debye-Scherrer geometry for x-ray powder diffraction (common for synchrotrons). 4. A new polarization function has been added (#4). This is to be used for image plate data where the intensity is not conserved (in Fit2D parlance). Thus, the calculated pattern requires an additional scaling by sin(2-theta) to correct for the increase with 2-theta owing to the longer integration arc. The names of Lp corrections #0 and #1 have been changed to reflect their true nature. May 17, 2004 1. A new capability has been installed in GSAS. It will do a multipattern Pawley refinement against a single list of reflections. The reflections can be from more than one phase and the patterns can be subject to refinements where some phases are done by Rietveld and others by Pawley (not tested). The Pawley set of reflections can also be subject to a restraint against negative values. For conventional CuKa data a single reflection entry is used to represent both Ka1 and Ka2 peaks; the RATIO parameter is used to scale them appropriately. The results are in the EXP file so they can easily be used for other purposes. Be aware that POWPREF will wipe out results from previous Pawley refinements and that much the same care in beginning refinements after POWPREF when doing LeBail refinements also must be done with Pawley refinements. May 7, 2004 1. The Finger, Cox Jephcoat peak shape functions have been improved in precision speed (Thanks to Larry Finger). They now do a better job on very low angle peaks and are ~50% faster. 2. A bug in the constraint editing for profile functions has been fixed. It used to prevent one from entering a constraint for profile parameter #21 or larger.
Le Bail - Gof
I am using the software Quanto to quantify some simple patterns, mixes of calcite and quartz. I read a lot of information about Rietveld, but I have not enough knowledgments about crystal structures, PO, etc. I refined a pattern of 15% of quartz and 85% of calcite. I got a Gof of 1.9 using Model in the Structure Factor Calculation Procedure (SFCP), but the peak shapes were poorly described (I changed the peak-shape functions and they did not change), but the process got convercence. Then I put the Le Bail option in the SFCP, I got a Gof of 1.1 (the peak-shapes were better than in Model), but the process did not get convergence. In both cases, I put the G parameters to correct the PO. WHY?
Re: Problem of low-angle
Dear Stephen, I apologise for calling you chuisy last time round. One suggest to test the size of the footprint is, I think, to use common rock salt. Grind this up and place liberally over your plate and you can check at low angle where the X-rays are striking the plate, because the salt will go dark? This should give an idea of how the incident beam changes with slits. Don't quote me on this as I haven't tried it. A nickel filter will remove most CuKbeta. I don't know what source you are using, but is there any risk of fluorescence, because if you are having problems with the slits, regardless of any precautions you take to remove fluorescence, if the slits are not correctly configured then a raised background may be noticed at low angle anyhow. Regards William Bisson On Fri, 4 Jun 2004 [EMAIL PROTECTED] wrote: Dear William, Thank you for your kind suggestion. Well, our XRD is not a very new model, we use a fixed divergent slit, receving slit and antiscattering slits. But I need to check them whether the divergent and anti-scattering slits are in the same sizes. When I added the anti-scatter slit, the bump did reduce in size but still there. In this case, can you teach me what precautions we need to concern when the Ni-filter, beam mask, sample holder (glass plate) and the receiving slits are used? I will make some reading as well, however I do willing to hear some practical experience from XRD users. many thanks, stephen chui Quoting William Bisson [EMAIL PROTECTED]: Dear Chuisy, The broad bump you are experiencing at low angle may be to do with the slits. One has to be careful with these new XRD machines which have variable slits, to make sure the divergent and the anti-scattering slits are equal and if you choose to keep the slits fixed, which usually results in a broad peak, the footprint is small. However, this will result in poor high angle data. It does require a deal of playing around with the slits to see what each of the effects are. The main purpose of variable slits is to have consistant data throughout a range of 2Theta, which it will do, however, it does make refinement just that little harder, having to take into account changing slit width over 2Theta. If anyone has any good tips on this matter, I would be very interested to know. Regards William Bisson On Thu, 3 Jun 2004 [EMAIL PROTECTED] wrote: Dear all, We are recently installing a XRD, and there is a huge peak occurred in the 3-7deg (2T) which is masking the useful signals from the sample. How can we eliminate the big peak there? We use 40kW, 30mA CuKa x-ray, divergent and receving slits are used, many thanks, stephen Chui
