Re: Combined neutron/x-ray refinements
I would like to add my two cents to the discussion on ESDs and
weightings.
Paolo wrote:
> Clearly, the ESD on x1 have worsened in this simple case. This does not
> prove the general case, but there might be a proof for that as well.
This conclusion was reached by applying non-linear least squares to
solving for x1 and x2 in the following:
^I1=x1+x2; I2=x1+2*x2, I3=x2, and assuming unitary weights.
ESDs squared were calculated as
2 and 2/3 for x1 and x2 respectively:
and 5 and 2 for x1 and x2 respectively excluding I3
This change in ESDs makes perfect sense to me. Excluding I3 lessens the
certainty in determining x2 which in turn lessens the certainty in
determining x1. My conclusion is that there is nothing wrong with the
ESDs calculated.
Now, for x1 and x2 to be independent of I3 then I3 should not contain
any parameters that are also in equations that are functions of x1 and
x2; for example:
I1 = x1 + x2
I2 = x1 + 2 x2
I3 = x3
I3 here is now independent of I3. And the A and B matrices becomes:
A = {{2, 3, 0}, {3, 5, 0}, {0, 0, 1}}
B = {{5, -3, 0}, {-3, 2, 0}, {0, 0, 1}}
Excluding I3 means excluding x3 as well; the A and B matrices here are:
A = {{2, 3}, {3, 5}}
B = {{5, -3}, {-3, 2}}
As you can see that in both cases the ESDs for x1 and x2 are the same.
Some other points I would like to clear up,
> 1) You construct the Aij matrix (also sometimes called the Hessian matrix)
>
> Aij=D^2chi^2/DxiDxj (D is the partial derivative sign. E-mail is
> still too primitive)
The term Hessian matrix refers to the second order terms obtained when
expanding a function in a Taylor series.
Typically Chi^2 is written as:
Chi^2 = Sum[ w(i)^2 (Io(i) - Ic(i))^2 , i]
where Io(i) an the observed data point
Ic(i) a calclated data point
w(i)^2 the weighting term for data point I
and the summation is over i
Ic(i) is expanded to a first order Taylor approximation, or:
Ic(i) = Ic(i, P) + Sum[ dIc(i)/dP(p) Del(p) , p]
where P is the parameter vector
Del(p) the change in parameter P(p)
dIc(i)/dP(p) is the derivative of the Ic(I) wrt parameter P(p)
Chi^2 becomes
Chi^2 = Sum[ w(i)^2 (Io(i) - Ic(i, P) - Sum[ dIc(i)/dP(p) Del(p) ,
p])^2 , i]
Differentiating Chi^2 wrt to each parameter P(p) and equating these
equation to zero yields the normal equations which in matrix form looks
like:
A Del(P) = Y
where Aij = Sum[ w(i) dIc(i)/dP(i) dIc(i)/dP(i) , i]
These equations are then solved for the changes in the parameters
Del(P). This term for Aij is different to the one given by Paolo and is
applicable to single crystal data. It is also applicable to powder data
by replacing i by 2Th and I by the intensities as a function of 2Th.
Thus I do not know where the following term by Paolo came from:
^ Aij=D^2chi^2/DxiDxj
Note that it is sometimes useful to expand Chi^2 itself but not in the
analysis of XRD data.
Alan
RE: Combined neutron/x-ray refinements
Jon Wright wrote : >PS: Any offers other than GSAS and multipattern fullprof for actually >doing these fits? Yes ARITVE, for amorphous compounds : http://www.cristal.org/aritve.html Nevertheless, ARITVE can work also for crystallized compounds with simple profile shape (gaussian only), and no zeropoint, and etc : do not try ! I have tested a combined Rietveld refinement on calculated neutron and X-ray data for CuVO3. Good news, the neutron data inclusion does not seem to degrade the X-ray result ;-). However ARITVE works by using f/ values, so that the X-ray data looks like neutron ones : no intensity decrease at large angle... Best Armel Le Bail - Université du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France http://www.cristal.org/
Re: [RE: Combined neutron/x-ray refinements]
OK, so iff the structure can be properly described by both datasets, the main problem that we have is what to do with the esd's and chi**2. It seems that the best thing is for the refinement software to give individual values for each refinement and then an overall value (perhaps rescaled). At least then when we look at a publication we can properly examine the quality of a refinement which is of course what we want. Bob (von Dreele) and Juan (Rodriguez-Carvajal), do you have any comments on this if you are listening? -Andrew Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.
RE: Combined neutron/x-ray refinements
Well, it looks like my "feeling" might have been wrong this time. The way you may want to go through the "tedious" proof is the following. 1) You construct the Aij matrix (also sometimes called the Hessian matrix) Aij=D^2chi^2/DxiDxj (D is the partial derivative sign. E-mail is still too primitive) In the simple case of a single crystal this is often approximated as Aij=Sum Wh*DIh/Dxi*DIh/Dxj 2) You invert the matrix: Bij=(Aij)^-1 3) Assuming chi^2=1, the ESD are the square roots of the diagonal elements of Bij. You can do this for very simple cases, say with 3 reflections and 2 parameters: I1=x1+x2; I2=x1+2*x2, I3=x2, and assuming unitary weights. Taking into account all 3 reflections, you get: 2 -1 Bij= -1 2/3 Excluding I3 (which is insensitive to x1) you get 5 -3 Bij= -3 2 Clearly, the ESD on x1 have worsened in this simple case. This does not prove the general case, but there might be a proof for that as well. Sorry about the confusion. Paolo
RE: Combined neutron/x-ray refinements
On Tue, 25 May 1999, Alan Hewat, ILL Grenoble wrote: > > Mainly because the ESD's are only correctly calculated if the model > is CAPABLE of fitting the data. This is not usually true when systematic > errors are important compared to statistical errors, since the model is > usually not capable of describing these systematic errors fully - > background, texture etc... > ...and weights. Lubo
RE: Combined neutron/x-ray refinements
On Tue, 25 May 1999, Alan Hewat, ILL Grenoble wrote: > >>I guess the degradation which is found would come from parameters which > >>are determined by both datasets and come out with different values in each > >>separate refinement. > > If they come out differently it is because they are differently biased by > different systematic errors in the data not described by the model. I was thinking of C-H (D) bondlengths from x-ray and neutron data. Don't they come out differently if you use spherical form factors for the x-ray data? I guess a neutron expert might look on this as an systematic error in the x-ray model :) Maybe not if one looks on the x-ray refinement as fitting of the electron density function, rather than the nuclear positions. For bonding studies it is the differences which are of interest! Jon Wright. PS: Any offers other than GSAS and multipattern fullprof for actually doing these fits?
RE: Combined neutron/x-ray refinements
On Tue, 25 May 1999 [EMAIL PROTECTED] wrote: > Not necessarily. In order to get the ESD, the variance-covariance matrix is > multiplied by chi^2, and the roots of the diagonal elements are taken. The justification for multiplying by chi^2 is to assume that the systematic errors are really just due to overestimated counting statistics and you can rescale the weight of each data point accordingly. A question arises as to whether you should rescale each pattern's esds according to the individual patterns chi^2 or do you have to use the overall chi^2 for both together? Thinking of an (over-determined) D20 data and an (under-determined) lab x-ray data set then it makes sense to rescale errors for the D20 data but not the x-ray (common sense?!). It seems as if the method for calculating the esd's is nonsense - surely one can only justify rescaling the weights on a per dataset basis. The systematic errors which are being accounted for in each dataset are different. Fullprof (multipattern) does give a chi^2 per pattern although I don't know how it gets the esd's, GSAS doesn't so I assume it degrades the esd's. (I read that the multiplication by chi^2 has no basis in statistics anyway :) So is it compulsory to multiply by the overall chi^2? If not then I see no reason for a degradation unless the individual fits get worse due to a disagreement over a parameter. > Therefore, if the chi^2 of the combined refinement is worse than that of the > individual ones, the ESD will automatically be worsened. I think this is by > far the commonest case. Agreed although I'm interpreting it as an odd method for estimating an error. Is it set in stone? > Also, by adding reflections that are insensitive to > a given parameter my feeling is that you increase the esd on that parameter > even if chi^2=1, but the proof of this is too tedious. Can you direct me to a text with this tedious proof? My feeling is that if the derivative of a data point w.r.t a parameter is small or zero then it does not affect the LSQ calculation unless it alters the chi^2. If the chi^2 is 1 then how do an extra bunch of zero derivatives affect an esd??? For example adding or excluding background regions shouldn't alter the esd's on positions provided the chi^2 is unchanged. Is there anything other than GSAS for doing combined fits anyway? Apologies to the list if I am displaying my ignorance, sometimes it's the quickest way to learn. Jon Wright PS: Sorry to pick at your comments Paolo, it's a shame I'm not at RAL at the moment. Could have discussed it out over a coffee...
RE: Combined neutron/x-ray refinements
>>I guess the degradation which is found would come from parameters which >>are determined by both datasets and come out with different values in each >>separate refinement. If they come out differently it is because they are differently biased by different systematic errors in the data not described by the model. >Not necessarily. In order to get the ESD, the variance-covariance matrix is >multiplied by chi^2, and the roots of the diagonal elements are taken. >Therefore, if the chi^2 of the combined refinement is worse than that of the >individual ones, the ESD will automatically be worsened. You may also get a higher chi^2 with higher resolution data, so does that mean that the structure will be less well determined with hi res data ? I think not, because the correlation between structural parameters should then be smaller - even if you have more points to fit with the same number of parameters, and the peak shapes are less well described by the model. You should similarly do better if you have both X-ray and neutron data (in the absence of bias). >Also, by adding reflections that are insensitive to >a given parameter my feeling is that you increase the esd on that parameter >even if chi^2=1, but the proof of this is too tedious. Tedious and also impossible ? (This sounds like a contradiction in terms) There is no case you can make based on pure statistics or the mathematics of refinement. The only way combined refinement can be worse is if you introduce bias through systematic error (which unfortunately may happen). >... in most cases that the ESD's are underestimated. Mainly because the ESD's are only correctly calculated if the model is CAPABLE of fitting the data. This is not usually true when systematic errors are important compared to statistical errors, since the model is usually not capable of describing these systematic errors fully - background, texture etc... The conclusion is that you should use combined refinements provided that one set of data does not contain important uncorrected systematic errors. Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
RE: Combined neutron/x-ray refinements
As chi^2 is a function of the number of data points included in the refinement, combined refinements have considerably improved values for a total chi^2 when compared with refinements carried out against individual data sets. Correspondingly the ESDs in the combined refinement output should be significantly lower than those obtained from a single data set refinement unless there is something drastically wrong with the application of combined refinement to the particular problem (e.g. preferred orientation, surface vs bulk etc). It is my experience that the combined refinement chi^2 is always lower than that obtained from using just (say) the neutron data. We have frequently collected data sets at both room temperature and 5 K using D2b. The room temperature data are refined simultaneously with lab X-ray data to give a chi^2 of 2.02 whilst the D2b data collected at 5 K refined as a single data set gives chi^2 of 4.53 (published in JACS, 1999, 121, 3958-3967). In my experience this improvement in chi^2 is typical. Eddie Cussen Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR United Kingdom E-mail: [EMAIL PROTECTED] tel: (..44)(0)1865-272602 Fax: (..44)(0)1865-272690 On Tue, 25 May 1999 [EMAIL PROTECTED] wrote: > Jon Wright wrote: > > >I guess the degradation which is found would come from parameters which > >are determined by both datasets and come out with different values in each > >separate refinement. > > Not necessarily. In order to get the ESD, the variance-covariance matrix is > multiplied by chi^2, and the roots of the diagonal elements are taken. > Therefore, if the chi^2 of the combined refinement is worse than that of the > individual ones, the ESD will automatically be worsened. I think this is by > far the commonest case. Also, by adding reflections that are insensitive to > a given parameter my feeling is that you increase the esd on that parameter > even if chi^2=1, but the proof of this is too tedious. > > Paolo >
RE: Combined neutron/x-ray refinements
Jon Wright wrote: >I guess the degradation which is found would come from parameters which >are determined by both datasets and come out with different values in each >separate refinement. Not necessarily. In order to get the ESD, the variance-covariance matrix is multiplied by chi^2, and the roots of the diagonal elements are taken. Therefore, if the chi^2 of the combined refinement is worse than that of the individual ones, the ESD will automatically be worsened. I think this is by far the commonest case. Also, by adding reflections that are insensitive to a given parameter my feeling is that you increase the esd on that parameter even if chi^2=1, but the proof of this is too tedious. Paolo
Combined neutron/x-ray refinements
Hi all, Am I right in thinking there are roughly two camps in this dicussion? Those who think that adding more data degrades the refinement if that data is not useful and those who think it makes no difference. (I say 'not useful' in the context of Vanadium in neutron data or deuterium in x-ray data for examples). My understanding is that the least squares matrix is made up of the derivatives of the differences between data and model w.r.t the parameters you are refining. So if a dataset has no useful information for a parameter then the relevant derivatives are zero. So there should be no degradation in the esd on the parameter, right? This would imply both datasets should (always) be used simaltaneously. I guess the degradation which is found would come from parameters which are determined by both datasets and come out with different values in each separate refinement. The higher esd reflects the disagreement between the neutrons and x-rays. (Now enter the questions about precision and accuracy.) Is this the area where there is a disagreement? What should one do when the datasets disagree with each other? Ideally work out why and model that in a combined fit! In practice either -Do a combined fit and report postions with higher esd's (as you aren't too sure where the atoms really are.) -Do two separate fits, each having lower esd's but disagreeing with each other. Which is better? Enough from me now, it's time to do some work today. Incidentally does anyone have an example of a refinement where parameters are degraded by the combined fit *and* they agree with each other when two separate fits are carried out. Jon Wright PhD Student, Chemistry Dept, Cambridge Uni, UK. On 25 May 1999, Andrew Wills wrote: > Alan, > > I am not suggesting removing reflections. But, I think that we should make > sure that we are combining the data in the best possible way. If we know have > strong information on a vanadium position from X-rays and (extrapolate again) > have only noise from neutrons, then stastically introducing the neutron data > whilst no changing the best fit will degrade the least - squares approach to > it. The final structure should fit all data, but are we approaching it > optimally? I know that this is a can of worms, but it is good to think about > what we are doing as combined refinements will continue to become less exotic. > > -Andrew > -- > Andrew Wills > Centre D'Études Nucléaires de Grenoble > > > "Alan Hewat, ILL Grenoble" <[EMAIL PROTECTED]> wrote: > >If we have an atom that is seen by one > >radiation and not by the other there will be a degradation in the quality of > >the parameters by combining the refinement in the current fashion. > > Do you mean for example that we might degrade the parameters of a V atom > by introducing neutron data ? > > I don't think this is true, but it is an interesting question. If we were to > extrapolate this argument "ad absurdum" we could say that because some > reflections (for a given radiation) do not give any information about some > parameters (easy to demonstrate) then we would obtain better estimates > for those parameters by removing those reflections from the least squares > process. (Surely untrue :-)
RE: Combined neutron/x-ray refinements]]]
Oops, forgive the typos! I haven't found a coffee yet :-) Andrew Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.
Re: [Re: [RE: Combined neutron/x-ray refinements]]
Alan, I am not suggesting removing reflections. But, I think that we should make sure that we are combining the data in the best possible way. If we know have strong information on a vanadium position from X-rays and (extrapolate again) have only noise from neutrons, then stastically introducing the neutron data whilst no changing the best fit will degrade the least - squares approach to it. The final structure should fit all data, but are we approaching it optimally? I know that this is a can of worms, but it is good to think about what we are doing as combined refinements will continue to become less exotic. -Andrew -- Andrew Wills Centre D'Études Nucléaires de Grenoble "Alan Hewat, ILL Grenoble" <[EMAIL PROTECTED]> wrote: >If we have an atom that is seen by one >radiation and not by the other there will be a degradation in the quality of >the parameters by combining the refinement in the current fashion. Do you mean for example that we might degrade the parameters of a V atom by introducing neutron data ? I don't think this is true, but it is an interesting question. If we were to extrapolate this argument "ad absurdum" we could say that because some reflections (for a given radiation) do not give any information about some parameters (easy to demonstrate) then we would obtain better estimates for those parameters by removing those reflections from the least squares process. (Surely untrue :-) What is true is that if we introduce systematic errors by combining radiations, we may indeed degrade the result. For example if we have serious preferred orientation with a very small X-ray sample, it is probably unwise to introduce this biased information into the refinement of the neutron data, where there may be less bias because of the average over a much larger volume. But if the data is not biased, you must always (?) do better by including more data, with for example combined X-ray and neutron refinements. >Surely, it >would be better to use a new weighting function for the atomic parameters, >that is dependent on the scattering lengths for each radiation. Playing around with weighting schemes is to enter dangerous territory. Alan H. Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/ Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.
Re: [RE: Combined neutron/x-ray refinements]
>If we have an atom that is seen by one >radiation and not by the other there will be a degradation in the quality of >the parameters by combining the refinement in the current fashion. Do you mean for example that we might degrade the parameters of a V atom by introducing neutron data ? I don't think this is true, but it is an interesting question. If we were to extrapolate this argument "ad absurdum" we could say that because some reflections (for a given radiation) do not give any information about some parameters (easy to demonstrate) then we would obtain better estimates for those parameters by removing those reflections from the least squares process. (Surely untrue :-) What is true is that if we introduce systematic errors by combining radiations, we may indeed degrade the result. For example if we have serious preferred orientation with a very small X-ray sample, it is probably unwise to introduce this biased information into the refinement of the neutron data, where there may be less bias because of the average over a much larger volume. But if the data is not biased, you must always (?) do better by including more data, with for example combined X-ray and neutron refinements. >Surely, it >would be better to use a new weighting function for the atomic parameters, >that is dependent on the scattering lengths for each radiation. Playing around with weighting schemes is to enter dangerous territory. Alan H. Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
Re: [RE: Combined neutron/x-ray refinements]
Dear All, Firstly, it was a pleasure to return to my email and read an interesting discussion on combined refinements. It is good to have aired some of the problems and limitations (e.g. are neutrons and X-rays seeing the 'same' sample?). With a simplistic view, this technique must be the way ahead as we are adding together more independent information. A good question to raise now, is how we should do it best. If we have an atom that is seen by one radiation and not by the other there will be a degradation in the quality of the parameters by combining the refinement in the current fashion. Surely, it would be better to use a new weighting function for the atomic parameters, that is dependent on the scattering lengths for each radiation. Ignoring the systematic errors that differ between neutrons and X-rays , perhaps the question that should be raised is how to best combine the information that each dataset holds. -Andrew --- Andrew Wills (Dr) Centre D'Études Nucléaires de Grenoble p.s./ Tao, the easiest way to add a list of atoms to a GSAS phase is by manual editing of the .exp file. I have a little program (DOS) that does this if you are interested. Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.
Re: Combined neutron/x-ray refinements
On Armel's point on using the internet. The big microscopy centres seem to be going big time for TelePresence/Collaboratories where collaborators and users can routinely use the internet rather than expensively hopping on and off planes to interactively make use of the eqiupmen: http://tpm.amc.anl.gov http://www-ncmir.ucsd.edu/CMDA/ http://www.uq.edu.au/nanoworld/online.html Also, high reliability automatic sample changers are routine in laboratory XRD systems and are great time savers for both users and equipment staff. An example with a Philips X'PERT system using a 42 sample PW1775 sample changer: a user was able to load 42 samples for long Rietveld data collections and go on two weeks holiday leave while the data collected. (Though I did check the XRD each day on their behalf). Though what synchrotron/neutron facilities have such an option (getting up at ~2am to change a sample over is not that much fun)? (From memory, there was a rotary sample changer on Bob von Dreele's neutron powder diffractometer?) Lachlan. > Jaap wrote : > > >With regard to the "brown envelope" technique. I am not such a fan of > >that. It is very hard for a beam line scientist the know whether the data > >collected are as desired, have a good enough s/r ratio etc, which details > >to look for. In addition, the last few years when we went we had more > >samples that we good run sensibly. For that reason I use 'on the fly' > >refinements to check whether things are going OK. With respect to the > >additional costs, what are the daily running costs of say D2b or HRPD, > >10,000-15,000 dollar orso? The travel costs are than a relative small part > >of the total cost involved. > > Arguments rejected ;-). We are at Internet times. Your professional > eyes need to see the powder pattern ? OK, then install a WebCam > on the instrument (less than 100 US $), and discuss by WebPhone > with the engineer or technician in charge of the measurement. And > you can do it from the beach, diving into that glorious caribean sea, > though washing your soul of all your recent sins. Of course, if you > want to walk on the mountain at Grenoble, this idea will seem > ridiculous. Furthermore, the travel cost and all costs may be at the > charge of the Facility, when a proposal is accepted. If the labs had > to pay, there will even be less external neutron users than now. Does > NIST take in charge all expenses as ILL do ? > > 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/ > -- Lachlan M. D. Cranswick Collaborative Computational Project No 14 (CCP14) for Single Crystal and Powder Diffraction Daresbury Laboratory, Warrington, WA4 4AD U.K Tel: +44-1925-603703 Fax: +44-1925-603124 E-mail: [EMAIL PROTECTED] Ext: 3703 Room C14 NEW CCP14 Web Domain (Under heavy construction): http://www.ccp14.ac.uk
Re: Combined neutron/x-ray refinements
Jaap wrote : >With regard to the "brown envelope" technique. I am not such a fan of >that. It is very hard for a beam line scientist the know whether the data >collected are as desired, have a good enough s/r ratio etc, which details >to look for. In addition, the last few years when we went we had more >samples that we good run sensibly. For that reason I use 'on the fly' >refinements to check whether things are going OK. With respect to the >additional costs, what are the daily running costs of say D2b or HRPD, >10,000-15,000 dollar orso? The travel costs are than a relative small part >of the total cost involved. Arguments rejected ;-). We are at Internet times. Your professional eyes need to see the powder pattern ? OK, then install a WebCam on the instrument (less than 100 US $), and discuss by WebPhone with the engineer or technician in charge of the measurement. And you can do it from the beach, diving into that glorious caribean sea, though washing your soul of all your recent sins. Of course, if you want to walk on the mountain at Grenoble, this idea will seem ridiculous. Furthermore, the travel cost and all costs may be at the charge of the Facility, when a proposal is accepted. If the labs had to pay, there will even be less external neutron users than now. Does NIST take in charge all expenses as ILL do ? 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/
Re: Combined neutron/x-ray refinements
On Tue, 11 May 1999, Armel Le Bail wrote: > I already suggested to install such an "automatic" powder diffractometer > at ILL. As Alan wrote recently, this could be a question of manpower. > I think that this is rather a local political question : it is not a very > interesting job for a human being to be a simple sample loader... A scheme like this could open up NPD to a wider range of users which would inevitably mean new users...? The simple loader might get involved in a lot of collaborations by returing refinement results instead of just raw data. They would also be in a unique position to offer the odd favour here and there. So long as it doesn't mean getting up every six hours then I think it sounds like a fantastic job. In any case - since simaltaneous fitting is being discussed - I have a question about multipattern fullprof which is now available as a beta version. It appears you can assign the statistical weight to each pattern, and in the example x-rays and neutrons are weighted equally. I'm wondering what the best choice is. Each observation comes with a statistical weight from the experiment? Isn't it a sin to alter that? Jon Wright PhD Student, Dept. of Chemistry, Lensfield Road, Cambridge, CB2 1EW, UK
RE: Combined neutron/x-ray refinements
Dear All, here is my (biased) opinion on the whole matter. 1) Intensity data: neutron powder diffraction *always* yield better intensity data than x-ray powder diffraction, including synchrotron. Contrary to popular belief, this is true not only for mixtures of heavy and light atoms, but also for all light or all heavy atoms. An interesting example of this was given by Bill David with his spherical harmonic refinement of C60 (a very fair test for x-rays). HRPD data yielded correct harmonic components up to l=18 (this was subsequently confirmed by a sigle-crystal XRD measurement). For synchrotron data, even the first component had the wrong sign (!). The exact reasons of this superiority have never been precisely quantified (to my knowledge), but it is generally believed to be due to the smaller amount of systematic errors. Preferred orientation has already been mentioned, but another big source of systematic errors in synchrotron XRD is powder statistics, because the beam is highly collimated on a small sample. You just need to take a look at the oscilloscope while the sample is spinning to understand what I am talking about. The advent of hard x-ray beamlines is not easing the problem: true, the beams are more penetrating, but you are selecting even fewer grains because the reciprocal space is compressed. 2) Atomic contribution. In general, x-ray data are dominated by the heavy atoms. This is not necessarily always a disadvantage, and, in fact, it can be a big advantage for complex structures. Here, the classic example is drug structure solution, where neutrons are hopeless just because they see too much. 3) Signal-to-noise ratio: at the moment, x-rays give far better STN ratios than neutrons. This means, that they are better at identifying weak features (e.g., weak superlattice peaks), even when have large light atom contributions. 4) Resolution: many neutron diffractometers have sufficient resolution so that this is not an issue in structural refinements, because peak widths at high q are usually sample-limited. The exceptions are large low-symmetry unit cells, where you can start telling the difference, say, between HRPD and D2B in terms of refinement stability. However, this assessment changes dramatically when you start talking about indexing and strain analysis. Here, only HRPD can compete with synchrotrons. 5) Complementary vs. simultaneous use: I personally think that there are many more cases for complementary use than for simultaneous use. This is certainly the case of my own work on perovskite superlattices, where I have published synchrotron and neutron data in the same paper many times, but I have never published a single simultaneous refinement. Not that I have never tried: in fact, I have several published examples of simultaneous refinements of two neutron wavelengths. However, when I tried to add the x-rays, I almost invariably spoiled the neutron results. I used the x-ray data to determine the strain model and to extract weak superlattice peaks. At the other extreme (small organic molecules) there is also a good case for complementary use, where x-rays are used to refine the non-hydrogen positions, which are then fixed in the neutron refinements to get the hydrogen positions. In general, I believe that simultaneous refinements are not worth the effort unless there is a clear reward in sight, like in the case of contrast variations to get occupancies in multiple solid solutions. 6) Accessibility. Armel's argument sounds a lot like the Aesop's fox (nolo acerbam sumere=these grapes are too sour), with the important difference that Armel could jump a little higher if he really wanted to get to the neutrons. Accessibility is an issue, but it can only be addressed if there is a consensus on the need to use the technique. I am absolutely sure that if enough chemists make enough noise that they want to measure their hot sample one week after they synthesized, they are going to get a rapid NPD service. And there need not be a human sample changer, you know. We've got machines for that. Best Paolo
Re: Combined neutron/x-ray refinements
> With regard to the "brown envelope" technique. I am not such a fan of > that. I have mixed feelings about the "brown envelope" technique. I do think that traveling to collect data that are run under routine, semi-automatic, conditions can be a waste of time and money, so Armel has a very good point. But like all the other neutron centers, but we are not staffed to run a mail-order shop. Further much work does really does benefit from having the experimenter here to learn 1) how things work and 2) to fiddle with the program of data collection (no battle plan survives contact with the enemy). A lot of previous discussions have centered around recognizing (and where possible proper treatment of) experimental artifacts in data. When instruments are virtual, who will know how to recognize the problems. So yes I agree that, > my presence > at Washington together with my samples would be a plus. Dr. Jaap Vente wrote: > Now I will go to the beach and dive into that glorious caribean sea and > wash my soul of all my recent sins. Sigh
Re: Combined neutron/x-ray refinements
What Armel is describing is precisely the way we started. We used it when required, discussed over a few pints whether it was a good idea or not, and came to the conclusion that it was. Later we tried it also on refinements where the need was not so clear, and now it is routine. I have not read any real arguments against it. But may be I am blind to that. Further, my remarks related the oxides were more directed at the precision/esd of the oxide position. Of course I do know where the oxides are within say 0.05A but i want it an order of magnitude better. Remember that most structural distortions in metal oxides are due to displacements of the oxide sublattice and often you fail to observe that with X-rays. With regard to the "brown envelope" technique. I am not such a fan of that. It is very hard for a beam line scientist the know whether the data collected are as desired, have a good enough s/r ratio etc, which details to look for. In addition, the last few years when we went we had more samples that we good run sensibly. For that reason I use 'on the fly' refinements to check whether things are going OK. With respect to the additional costs, what are the daily running costs of say D2b or HRPD, 10,000-15,000 dollar orso? The travel costs are than a relative small part of the total cost involved. > refinement was really fruitful (speaking of V atoms comes certainly to > mind, but why not prepare an isomorphous sample without V ?-). Because you want to study the V containing sample of course! Now I will go to the beach and dive into that glorious caribean sea and wash my soul of all my recent sins. Best Jaap Jaap Vente Cinvestav-IPN Unidad Merida Departamento de Fisica Aplicada Carretera Ant. a Progreso km 6 Apartado Postal #73 Cordemex Merida, Yucatan, 97310 Mexico Fax: (..) 52 - 99 - 812917 Tel: (..) 52 - 99 - 812960 ext 246/233 e-mail: [EMAIL PROTECTED]
Re: Combined neutron/x-ray refinements
Alan Hewat wrote: >Before some-one else says they can see hydrogen with x-rays, Mike >Glazer, Bill David and I saw vanadium with neutrons. So let's just >say "insensitive" but not "completely". Otherwise I agree :-) >Combined refinements are sometimes necessary and a good thing. > >1. David, W. I. F., Glazer, A. M. and Hewat, A. W. (1979) Phase Transitions 1, 155-69. >The structure and ferroelastic phase transition of bismuth vanadate (BiVO4). > I'd agree with Alan that neutrons are a little "insensitive" to vanadium. I've been looking at some vanadium substituted cuprates - in which the vanadium is tetrahedral as "VO43-". We got this information from 51V MAS-NMR - the 51V nucleus has spin I=7/2, a natural abundance of 99.76% and high relative sensitivity. Using this information we could set up the tetrahedral vanadium coordination environment in the model for the powder neutron data - allowing us to model the vanadium position with more confidence (if not precision). I'm no expert on 51V-NMR but there is a lot of stuff in the literature (e.g. O.B. Lapina et al., Progress in NMR spectroscopy, 24, 457-525, 1992) indicating that the degree of distortion around the 51V nucleus can be extracted from a detailed analysis of such NMR data. Lets assume we have near perfect "VO4" tetrahedra - it seems to me that a smart way to deal with the "insensitivity" of neutrons toward vanadium would then be to set up the the coordination around vanadium as a rigid body in GSAS with the bond angles fixed (initially) at 109o 28', and then allow the bond lengths to relax. That said, I can't actually figure out how to do this with GSAS though!! (But I'm persevering...) Best regards, Neil Hyatt. ** Neil Hyatt tel: +44-(0)121 414 4370 School of Chemistry fax: +44-(0)121 414 4442 University of Birmingham email: [EMAIL PROTECTED] Edgbaston Birmingham B15 2TT WWW: http://chemwww.bham.ac.uk/ UK
Re: Combined neutron/x-ray refinements
Toby wrote, >Armel you may indeed be more equal than some but, I would like to >welcome you to visit us in the suburbs of Washington, along with some of >your favorite samples. DC is no match for Paris or even Grenoble, and >our cafeteria does not come near the quality of the ILL, but there are >some other attractions. There is some misconception here too. You suggest that my presence at Washington together with my samples would be a plus. Thank you a lot. However, you are THE neutron specialist. I see the neutron diffractometers less than 3 days per year, so that my experience is lost each time. Sending samples by mail would not change so dramatically the result, reducing considerable the cost for the citizen tax payers... I already suggested to install such an "automatic" powder diffractometer at ILL. As Alan wrote recently, this could be a question of manpower. I think that this is rather a local political question : it is not a very interesting job for a human being to be a simple sample loader... This is related to a more general question about human relations between those preparing new samples (frequently they are chemists) and those having some power in characterizing them (frequently physicists). In spite of working in a chemistry lab, I am not sure to be neither a chemist or a physicist (I studied geology, geochemistry, mineralogy ;-). Anyway, I see at least one time per week some physicists of the neigbouring labs coming for trying "fishing" some new sample in order to give some food to their NMR, Mossbauer, EPR (etc) machines. They just need the sample, not the man having prepared it. Hope that this subject of discussion will keep you awake till Glasgow, Lubo. 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/
Re: Combined neutron/x-ray refinements
>this discussion has gone too far from the starting point. The question >really isn't "neutrons Yes/No", but if all of us have equal access to all >sources. People from both NIST, Grenoble or RAL would, no doubt, answer >yes, why not ? Well, if I were there I wouldn't hesitate for a moment, >but if I am outside, there are plenty of time and financial but NO >scientific constraints. I think the most of us would be happy if they >could access neutron sources as easily as their in-lab machines. >Do not agree, please. The days before GLASGOW are so boring ... Sorry, Lubo, but I agree. However, when you speak of easy access to a lab machine, you should account for some little problems that sometimes lead to long breakdown. Breakdowns are democratically distributed, even at ILL with the long reactor shutdown. Here with a brand new Bruker D8 powder diffractometer, we had not access to the machine longer than 3 months since December 1998, due to problems with bad ceramic tubes, random stop of the stepping motors, files not saved at the end of measurements and so on. Youth problems, may be. Have the happy buyers of the D8 similar problems (the Bruker mailing list if completely silent) ?? Armel 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/
Re: Combined neutron/x-ray refinements
On Mon, 10 May 1999, Armel Le Bail wrote: > published combined X-ray and neutron refinements. Am I so far > from the truth if I estimate the number of published works combining > X-ray and neutron in a single refinement to, say less than 20 cases ? I > would like to see, in the 10 next years, this number increase to, say, 1000, > for being convinced that this is really a way that should be adopted. > But I do not believe it. Such combined refinements will stay anecdotal > (or elitist if you prefer), reserved to specific cases, or to those having > easy access to both radiations. > Armel, this discussion has gone too far from the starting point. The question really isn't "neutrons Yes/No", but if all of us have equal access to all sources. People from both NIST, Grenoble or RAL would, no doubt, answer yes, why not ? Well, if I were there I wouldn't hesitate for a moment, but if I am outside, there are plenty of time and financial but NO scientific constraints. I think the most of us would be happy if they could access neutron sources as easily as their in-lab machines. Do not agree, please. The days before GLASGOW are so boring ... Best, Lubo
Re: Combined neutron/x-ray refinements
At 07:08 PM 5/10/99 +0200, Armel wrote: >PS- take the Rietveld Round Robin PbSO4 X-ray pattern and omit >the O atoms, you will have RB~15%. Make a Fourier difference >and you will see if the "light" atoms are so light, when using good data. >When I remember my crystallography courses 25 years ago, maybe, >I see the decomposition of the structure factor by atom pairs. O atoms >are involved not only in O-O pairs, but also in Pb-O and S-O pairs, >fortunately. In many cases, like Li2TbF6, or LiSbWO6 and so on, I had >no difficulty to see the Li atoms in the Fourier syntheses, by X-ray. >Hence not a large need of neutrons, but in a few cases. Armel could "see" the Li atoms from X-ray data, and I found the H in AlSiO3(OH) (Schmidt et al., American Mineralogist 83, 881, 1998) with powder data in a 3-phase mixture. It was clearly visible in the difference Fourier and could be refined with reasonable distances and thermal factors. I think all would agree that this was an unusual case, and that one would not normally expect to find H atoms from X-ray powder data. Clearly it is important to have a variety of tools in your arsenal and use whatever of them you need. In addition, make certain that your scientific administrators and legislators understand the need for neutron facilities! Larry -- Larry W. Finger [EMAIL PROTECTED] Geophysical Laboratory Phone: +1 (202) 686-2410 X 2464 5251 Broad Branch Road N.W. FAX: +1 (202) 686-2419 Washington, DC 20015-1305, USA http://www.gl.ciw.edu/~finger/ < Note NEW URL http://btgix8.bgi.uni-bayreuth.de/~lafi
Re: Combined neutron/x-ray refinements
Jaap wrote : >Finally Armel, in my view an attempt the split the Rietveld community in >two, i.e. in house X-ray and central facility neutrons, is artificial. By >teh way did you note that someone called Mark Weller was on your neutron >list as well. As far as I know, he has been working in Southampton for >some time now. So there is hope for ordinairy university based scientist >to use neutrons! But the question was about using both. I don't know if Mark Weller published combined X-ray and neutron refinements. Am I so far from the truth if I estimate the number of published works combining X-ray and neutron in a single refinement to, say less than 20 cases ? I would like to see, in the 10 next years, this number increase to, say, 1000, for being convinced that this is really a way that should be adopted. But I do not believe it. Such combined refinements will stay anecdotal (or elitist if you prefer), reserved to specific cases, or to those having easy access to both radiations. Nobody among the readers has already tried to combine both kind of data and finally preferred to publish the neutron data only, for avoiding pain and lightening the discussion ?-)). I will not send the first stone... I do have published separate neutron and X-ray refinements and compared the results (JSSC 89, 1990, 282-291, or Eur. J. Solid State Inorg. Chem. 15, 1988, 551-563). This is not really a problem to publish independent refinement on neutron and X-ray data. I do agree that in a few cases (may be the 20 cases cited above), a combined refinement was really fruitful (speaking of V atoms comes certainly to mind, but why not prepare an isomorphous sample without V ?-). Rare is not always beautiful. All the best, Armel PS- take the Rietveld Round Robin PbSO4 X-ray pattern and omit the O atoms, you will have RB~15%. Make a Fourier difference and you will see if the "light" atoms are so light, when using good data. When I remember my crystallography courses 25 years ago, maybe, I see the decomposition of the structure factor by atom pairs. O atoms are involved not only in O-O pairs, but also in Pb-O and S-O pairs, fortunately. In many cases, like Li2TbF6, or LiSbWO6 and so on, I had no difficulty to see the Li atoms in the Fourier syntheses, by X-ray. Hence not a large need of neutrons, but in a few cases. 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/
Re: Combined neutron/x-ray refinements
>The neutron fit will be >completely insensitive to the V positions and the x-rays insensitive to >the D positions. (This is easily verified.) Before some-one else says they can see hydrogen with x-rays, Mike Glazer, Bill David and I saw vanadium with neutrons. So let's just say "insensitive" but not "completely". Otherwise I agree :-) Combined refinements are sometimes necessary and a good thing. 1. David, W. I. F., Glazer, A. M. and Hewat, A. W. (1979) Phase Transitions 1, 155-69. The structure and ferroelastic phase transition of bismuth vanadate (BiVO4). And its not even in ICSD :-( Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
Re: Combined neutron/x-ray refinements
Armel Le Bail wrote: > I note that this is mainly the neutron community > that is not preoccupated by improving the positions of the > heavy atoms by using X-ray ;-). With a few exceptions, neutrons are sensitive to most heavy atoms. For many materials the improvement obtained using both x-rays and neutrons vs neutrons alone are minor, but for complex materials, or materials such as zeolites, that do not scatter well, use of only one radiation can result in a severely flawed result (examples on request). > However, I tend to think > that a simultaneous refinement could eventually degrade > the heavy atom position accuracy AND degrade the light > atoms accuracy. This is a common misconception. As a gedanken experiment, suppose you have a structure containing D atoms and V atoms. The neutron fit will be completely insensitive to the V positions and the x-rays insensitive to the D positions. (This is easily verified.) The only case where the positions are degraded is where there is a systematic experimental anomaly with one (or both) datasets. > Giving the results of 2 independent refinements seems better > to me. As supplementary material to demonstrate the combined refinement is valid? Sounds like a good idea to me! But, the combined refinement result will be the most accurate and thus the one to stick in the paper. > The confirmation of what I suspected : some peoples are > more equal than others !-). Armel you may indeed be more equal than some but, I would like to welcome you to visit us in the suburbs of Washington, along with some of your favorite samples. DC is no match for Paris or even Grenoble, and our cafeteria does not come near the quality of the ILL, but there are some other attractions. Brian H. Toby, Ph.D.Leader, Crystallography Team [EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562 voice: 301-975-4297 National Institute of Standards & Technology FAX: 301-921-9847Gaithersburg, MD 20899-8562
Re: Combined neutron/x-ray refinements
Let start to cut the crap and go to Armel's real reason not to use a combined refinement: > Of course this is only kidding and provocative opinion, as > usual. I love both radiations, indeed. However, I tend to think > that a simultaneous refinement could eventually degrade > the heavy atom position accuracy AND degrade the light > atoms accuracy. The former being better from the X-ray only > and the latter from the neutron only, not to speak of the various > problems that hurt me like, for instance : Do not agree! If you work on any transition metal oxide, what I am doing all the time, you will find that the oxide position are basically not determined. OK I work a lot with heavy elements like lanthanide's and iridium and the like. But in general the x-ray are very insensitive to where the oxides are. Not the forget the oxide content! So they do not effect the positions as required by the neutrons. Things can be even more true the other way around. e.g when you have a random dirtribution of either Ti or Mn and another metal you can have scattering lengths of zero. > - the need to refine two sets of cell parameters, inevitably > slightly different if both data are of high resolution. Basically what you mean is that the neutron diffractometer is not properly calibrated. Well a combined refinement in which you refine the neutron wavelength is than the answer! Recently I ran two different samples on D2b, and refined lambda: sample 11.59362(5) sample 21.59354(5) well that is very similar isn't it. I have read people stating on this e-mail list that they don't trust cell parameters after the third decimal. > - not exactly the same temperature True, but you can control that if you want to, and in general does bot seem to be the biggest pain. > - bulk with neutrons, surface with X-ray > What? Do you propose to throw all the single crystall structure determinations in the bin, out of the window or where else because it is a surface technique? Of course the penetration depth of x-ray is a lot smaller than that of neutrons, but it remains a bulk technique. Thank god other wise we would still be cleaving those large single crystals. I dare to say that you can study the bulk structure even with electrons, which have an even smaller penetration depth. Of course you have to be carefull as Brian Toby pointed out. But in those cases you either a non-homogeneous sample, or a phase transformation which does happen on the slow cooling central parts of the particles but not on the fast cooling outer regions of the particle. In those cased it is back to the preparation lab. > Giving the results of 2 independent refinements seems better > to me. But which journal is going to accept that! Finally Armel, in my view an attempt the split the Rietveld community in two, i.e. in house X-ray and central facility neutrons, is artificial. By teh way did you note that someone called Mark Weller was on your neutron list as well. As far as I know, he has been working in Southampton for some time now. So there is hope for ordinairy university based scientist to use neutrons! Best Jaap Jaap Vente Cinvestav-IPN Unidad Merida Departamento de Fisica Aplicada Carretera Ant. a Progreso km 6 Apartado Postal #73 Cordemex Merida, Yucatan, 97310 Mexico Fax: (..) 52 - 99 - 812917 Tel: (..) 52 - 99 - 812960 ext 246/233 e-mail: [EMAIL PROTECTED]
Re: Combined neutron/x-ray refinements
>The confirmation of what I suspected : some peoples are >more equal than others !-). Perhaps :-) But Armel, if you or some-one else is interested in setting up a neutron service in Grenoble as fast as at NIST, we would be very interested. We can provide the equipment (D1A is quite competitive with the machines at NIST :-) but we simply don't have the manpower to run it. Armel keeps telling me that X-rays are cheaper, but in reality *once you have the neutron source* there is no difference - the cost is almost all manpower. >The most productive neutron experts are 3x more >productive than the X-ray experts Anybody from the ILL member countries who can write a successful research grant for a couple of postdocs to run D1A can join the "most productive neutron experts". This is a serious offer ! BTW, people tend to forget that a high resolution neutron powder pattern, even on D1A, only takes ~6 hours, faster than most high- res. X-ray or synchrotron machines. Can you say "cost effective " ? Alan H. Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
Re: Combined neutron/x-ray refinements
Brian wrote: >Finally, I should mention in response to Armel that at least here at >NIST, most requests for time are scheduled within 2-8 weeks of when we >get them (see http://www.ncnr.nist.gov/~toby/bt1.html). The confirmation of what I suspected : some peoples are more equal than others !-). Looking at the two quite different X-ray and neutron communities applying the Rietveld method, I note that this is mainly the neutron community that is not preoccupated by improving the positions of the heavy atoms by using X-ray ;-). Few people are seen simultaneously in both lists below (those who would feel concerned by a double refinement). X-ray seems inaccessible to many neutron experts or at least lacking of any interest, and vice versa. The most productive neutron experts are 3x more productive than the X-ray experts, probably because the birth of X-ray Rietveld applications is more recent (~1981 with DBW, possibly). But it seems to me that Rietveld adaptation to X-ray was made by X-ray experts, because neutron experts did not care a lot about doing it. Why X-ray users should be interested in the precision on light atoms when neutron users don't seem to care about heavy atoms ? Of course this is only kidding and provocative opinion, as usual. I love both radiations, indeed. However, I tend to think that a simultaneous refinement could eventually degrade the heavy atom position accuracy AND degrade the light atoms accuracy. The former being better from the X-ray only and the latter from the neutron only, not to speak of the various problems that hurt me like, for instance : - the need to refine two sets of cell parameters, inevitably slightly different if both data are of high resolution. - not exactly the same temperature - bulk with neutrons, surface with X-ray Giving the results of 2 independent refinements seems better to me. Armel ==ICSD data=== http://pcb4122.univ-lemans.fr/icsd/icsdrndp.txt Authors versus numbers of Neutron Rietveld-refined structures in ICSD - release 1998/1 (REM=RVP AND REM=NDP : 3276 entries). -- 324 Fischer P 240 Jorgensen J D 189 Hewat A W 171 Hinks D G 125 Dabrowski B 115 Hitterman R L 110 Vogt T 108 Shaked H 107 Marezio M 105 Radaelli P G 103 Weller M T 101 Rodriguez-Carvajal J http://pcb4122.univ-lemans.fr/icsd/icsdrxdp.txt Authors versus numbers of conventional X-ray Rietveld-refined structures in ICSD - release 1998/1 (REM=RVP AND REM=XDP : 2557 entries). -- 122 Kanno R 107 Takeda Y 104 Kawamoto Y 80 Raveau B 79 Yamamoto O 76 Takano M 67 Hawthorne F C 66 Hervieu M 59 Michel C 54 Yamamoto T 51 Sato M
Combined neutron/x-ray refinements
I also use combined CW neutron and synchrotron refinements. A simple minded justification goes as follows. Most of the problems I work on are badly underdetermined -- at least by the crystallographic rule-of-ten (10 crystallographic observations for each structural variable). By changing scattering lengths, I get a second set of observations which gives me more observables. Thus, I agree strongly with all of Dr. Jaap Vente's points: > 1) in general the refinement is more stable. > 2) their is the possibility to study much more complicated structures > than with only one of the techniques. > 3) because you now have two really different sets of data your structural > model is more reliable. > 4) you can study compounds which contain elements that are difficult to > locate precisely with one technique, think of vanadium oxides or > manganese/iron oxides. Andrew Wills is correct that X-rays see the electronic distribution and neutrons see nuclei positions, but electrons distributions are pretty close to spherical (our form factors assume this) for high-Z elements and are usually well centered around the nucleus. One can make a good argument that displacement parameters (aka temperature factors) can be completely different for x-rays vs neutrons, but experimentally this is seldom true. In any case, for all but the simplest systems, with powder work we don't have the precision to tell. Besides, x-ray displacement parameters are pretty meaningless anyway :-). I do not know of any codes other than GSAS that do combined x-ray/neutron fits, but in GSAS all the experimental effects (orientation, absorption, etc) are segregated by dataset so one only needs to apply these corrections to the x-ray data. (Neutron data seldom have either problem). In any case, if you can't model them well, you can't use the data. The "weighting" problem is overstated. The data are weighted by how well you know them. Usually the x-rays do contribute more to the Chi2 than the neutron, but the algorithm will minimize the deviations in both appropriately. One could downweight the x-ray data artificially, since you will probably have worse precision on the more structurally accurate neutron data, but this will screw up the Chi2 value. The biggest problem for combined refinements is that you need to have exactly the same sample and the same conditions for both the x-ray and neutron work. Since single crystals are frequently grown under different conditions than bulk samples, the utility of combined x-ray single crystal - powder neutron refinements is limited. Alas, it is fairly common that someone makes a material, measures the x-ray diffraction and then scales up the synthesis for neutrons, but ends up with something different. Attempts to simultaneously fit one model to x-ray data from the first batch and neutron data from the second batch are a waste. Other issues can also arise. We recently had a case where a material seemed nearly pure by x-rays, but the neutron work showed that the centers of the large particles were still composed of unreacted starting material. The x-rays did not penetrate far enough to see the purity was only ~70%. It would probably be a good idea to check that the model obtained from the combined refinement agrees well with (possibly constrained) models using the individual datasets. Perhaps we could entice John Parise to write a message about how to do this. Finally, I should mention in response to Armel that at least here at NIST, most requests for time are scheduled within 2-8 weeks of when we get them (see http://www.ncnr.nist.gov/~toby/bt1.html). Brian H. Toby, Ph.D.Leader, Crystallography Team [EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562 voice: 301-975-4297 National Institute of Standards & Technology FAX: 301-921-9847Gaithersburg, MD 20899-8562
