Hi
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PDFgetX2, a GUI driven program to obtain PDF from X-ray powder diffraction
Hi Dear all, PDFgetX2 is a GUI driven program to obtain the total-scattering structure function S(Q) and the pair distribution function (PDF), G(r), from X-ray powder diffraction data. Its first beta release is available at http://www.pa.msu.edu/cmp/billinge-group/programs/PDFgetX2/ A brief user's manual is also provided with the tutorial data. In program PDFgetX2, a user-friendly GUI has been built to facilitate user interactions with data. Standard corrections due to background subtraction, sample absorption, polarization, Compton intensities are available. The final S(Q) and G(r) data files are multiple-column ASCII files with the processing parameters in the header, which can also be saved into a history file. Reloading of the history file will restore the full GUI section, and reproduce earlier data analysis. The the freely downloadable interactive data language (IDL) Virtual Machine, or the commercial (IDL) licensed distribution (version 6.0 or higher), is required for PDFgetX2 to run. Platform supported by IDL include Linux/UNIX, Windows, and MACINTOSH, and can be downloaded at http://www.rsinc.com/download/. Self-installers of PDFgetX2 are available for Linux/UNIX and Windows. We welcome everyone to try it out, and give us feedback, comments, and bug reports. Thank all, Xiangyun Qiu from Billinge group at Michigan State University
New to GSAS
Apu, 1. For now use the inst_xry.prm file in \gsas\examples. 2. POLA is appropriate only for X-ray data & not neutron data. GSAS knows the difference. Bob Von Dreele From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Mon 4/26/2004 11:45 AM To: [EMAIL PROTECTED] Hi, I am new to GSAS.I am facing some basic problem and I am confused. # I am using XRD data from Philips PW 1710 machine.How should I prepare the instrument parameter file? #Trying the examples I sometimes fond the option Refine POLA and sometime it was missing.Why it is so? Please help me. Thanks in advance. Regards, Apu Apu Sarkar Research Fellow Variable Energy Cyclotron Centre Kolkata 700 064 phone: 91-33-2337-1230 (extn. 3190) Fax: 91-33-2334-6871 INDIA
New to GSAS
Hi, I am new to GSAS.I am facing some basic problem and I am confused. # I am using XRD data from Philips PW 1710 machine.How should I prepare the instrument parameter file? #Trying the examples I sometimes fond the option Refine POLA and sometime it was missing.Why it is so? Please help me. Thanks in advance. Regards, Apu Apu Sarkar Research Fellow Variable Energy Cyclotron Centre Kolkata 700 064 phone: 91-33-2337-1230 (extn. 3190) Fax: 91-33-2334-6871 INDIA
Re: GSAS informations
Dear Bob and Jon, one reply from the public is: come to share your and to get other ideas to the meeting Size-Strain IV (http://www.xray.cz/s-s4/), a satelite workshop of the EPDIC-9 (http://www.xray.cz/epdic/), end of this summer in Prague. The thinks can be even more complex: The supperposition of narrow and broad peaks can come not only from the size-strain symmetry lower than Laue symmetry (Andreas's example for the polycrystal), but also from a non-homogeneous distribution of lattice defects (for example dislocations), even in the monocrystal. See you in Prague Radovan Von Dreele, Robert B. a écrit: Jon, I risk a public reply here. One possibility everyone should be open to is that a real phase change has occured during some experimental manipulation of your sample. Some phase changes are quite subtle and involve only slight (and at first sight) quite odd line broadening. Higher resolution study sometimes reveals a splitting of these peaks which is then taken as a sign of a phase change. However, without this the linebroadening is sometimes well described by various anisotropic models (and sometimes not!). Historically, one only need reflect on the work done over many years on various high Tc superconductors and their relatives to know what I mean. Andreas does have the right idea about random powders but solid polycrystalline materials (e.g. metal bars) are a different matter especially if they have been "worked" because the various crystallites are no longer in "equal" environments. Fortunately, the kind of stuff that happens in metals is generally much less of a problem i! n the other kinds of materials one studies by powder diffraction so models used in Rietveld refinements can be rather simplified. Bob Von Dreele From: Jon Wright [mailto:[EMAIL PROTECTED] Sent: Mon 4/26/2004 3:45 AM To: [EMAIL PROTECTED] >... to answer to your (too) long questions. May be later, OK? Going back to this quartics versus ellipsoids peak broadening stuff, maybe I can summarise: Why should the distribution of lattice parameters (=strain) in a sample match the crystallographic symmetry? If the sample has random, isolated defects then I see it, but if the strains are induced (eg: by grinding) then I'd expect the symmetry to be broken. Suggests to me the symmetry constraints should be optional, and that the peak shape function needs to know about the crystallographic space group and subgroups. Either the program or the user would need to recognise equivalent solutions when the symmetry is broken (like the "star of k" for magnetic structures). Why would anyone have anything against using an ellipsoid? That same function can be described by the quartic approach, it just has less degrees of freedom. In short, I don't understand why there is such a strong recommendation to use the quartics instead of ellipsoids or why the symmetry is not optional. I'm still persuing this because I have looked at something with a very small anisotropic broadening which seems to fit better with an ellipsoid which breaks the symmetry compared to a quartic which doesn't! Thanks for any advice, Jon -- Radovan Cerny Laboratoire de Cristallographie 24, quai Ernest-Ansermet CH-1211 Geneva 4, Switzerland Phone : [+[41] 22] 37 964 50, FAX : [+[41] 22] 37 961 08 mailto : [EMAIL PROTECTED] URL: http://www.unige.ch/sciences/crystal/cerny/rcerny.htm
Re: GSAS informations
Jon, I risk a public reply here. One possibility everyone should be open to is that a real phase change has occured during some experimental manipulation of your sample. Some phase changes are quite subtle and involve only slight (and at first sight) quite odd line broadening. Higher resolution study sometimes reveals a splitting of these peaks which is then taken as a sign of a phase change. However, without this the linebroadening is sometimes well described by various anisotropic models (and sometimes not!). Historically, one only need reflect on the work done over many years on various high Tc superconductors and their relatives to know what I mean. Andreas does have the right idea about random powders but solid polycrystalline materials (e.g. metal bars) are a different matter especially if they have been "worked" because the various crystallites are no longer in "equal" environments. Fortunately, the kind of stuff that happens in metals is generally much less of a problem i! n the other kinds of materials one studies by powder diffraction so models used in Rietveld refinements can be rather simplified. Bob Von Dreele From: Jon Wright [mailto:[EMAIL PROTECTED] Sent: Mon 4/26/2004 3:45 AM To: [EMAIL PROTECTED] >... to answer to your (too) long questions. May be later, OK? Going back to this quartics versus ellipsoids peak broadening stuff, maybe I can summarise: Why should the distribution of lattice parameters (=strain) in a sample match the crystallographic symmetry? If the sample has random, isolated defects then I see it, but if the strains are induced (eg: by grinding) then I'd expect the symmetry to be broken. Suggests to me the symmetry constraints should be optional, and that the peak shape function needs to know about the crystallographic space group and subgroups. Either the program or the user would need to recognise equivalent solutions when the symmetry is broken (like the "star of k" for magnetic structures). Why would anyone have anything against using an ellipsoid? That same function can be described by the quartic approach, it just has less degrees of freedom. In short, I don't understand why there is such a strong recommendation to use the quartics instead of ellipsoids or why the symmetry is not optional. I'm still persuing this because I have looked at something with a very small anisotropic broadening which seems to fit better with an ellipsoid which breaks the symmetry compared to a quartic which doesn't! Thanks for any advice, Jon
Re: GSAS informations
Dear Jon, Jon Wright wrote: >... to answer to your (too) long questions. May be later, OK? Going back to this quartics versus ellipsoids peak broadening stuff, maybe I can summarise: Why should the distribution of lattice parameters (=strain) in a sample match the crystallographic symmetry? If the sample has random, isolated defects then I see it, but if the strains are induced (eg: by grinding) then I'd expect the symmetry to be broken. Suggests to me the symmetry constraints should be optional, and that the peak shape function needs to know about the crystallographic space group and subgroups. Either the program or the user would need to recognise equivalent solutions when the symmetry is broken (like the "star of k" for magnetic structures). I would like to present my thoughts here, although some points overlap with points mentined previously by others: One has to think what one looks at: If you look at a single crystal grain, which may show, for whatever reason microstrain broadening (i.e. local distortions, e.g. due to dislocations) which is incompatible with the Laue group/crystal system symmetry. In powder diffraction, however, you look at an ensemble of crystallites, and the line broadening information is averaged. Even if you have a powder of identical crystals, each showing identical line broadening incompatible with Laue symmetry reflections of different width overlap because they have the same d-spacing. By that the line broadening of the crystals incompatible with Laue symmetry cannot be obtained directly from the powder pattern, e.g . by analysing the powder peaks' widths as a function of hkl, as you could do it for a single crystal. You can only analyse averaged widths. Thus you loose the decisive information. However, you may detect hkl dependent changes of the shapes of the reflections, e.g. superlorentzian peaks where, e.g. one broad and two narrow overlap (h00 reflection of a cubic crystals which show strong microstrain along [001] but low microstrain along [100] and [010]). It will be difficult to recognise such effects, unless they are really strong, and it may be even more difficult to interpret that, if you have not specific information about possible sources of the microstrain. The same problem of overlapping reflections of different widths is predicted for certain cases of quartic line broadening when the Laue group symmetry is lower than the crystal system symmetry (E.g. for the Laue class 4/m), as remarked by Stephens (1999). As much as I know, no such case has been reported yet. To summarise: I think refinement of anisotropic line broadening will be much more stable if constrained by symmetry, such that reflections equivalent by symmetry have the same width. One example related with that problem was presented at (I think it was a size broadening case, but similar conclusions may be valid for microstrain) Young, R. A., Sakthivel, A., Bimodal Distributions of Profile-Broadening Effects in Rietveld Refinement, J. Appl. Cryst. 21 (1988) 416 Why would anyone have anything against using an ellipsoid? That same function can be described by the quartic approach, it just has less degrees of freedom. Here I fully agree, and there ARE cases where theory predicts an ellipsoid (e.g. microstrain-like broadening due to composition variations) which should, however, obey the symmetry restrictions. If such a case is present an ellipsoid model should be used obeying the rule to use a minimum of refined parameters. On the other hand, one might imagine other cases, where you have ellipsoid broadening for the single crystals incompatible with symmetry and being then powder-averaged. However, this will be difficult to be recognised and interpreted (see above). In short, I don't understand why there is such a strong recommendation to use the quartics instead of ellipsoids or why the symmetry is not optional. I'm still persuing this because I have looked at something with a very small anisotropic broadening which seems to fit better with an ellipsoid which breaks the symmetry compared to a quartic which doesn't! Thanks for any advice, Jon I think it should be recommended first to use a minimum number of parameters to describe the line broadening, and if possible, secondly to use models which are mathematically compatible with the physics of the origin of the line broadening. Thus in some cases an ellipsoid model should be preferred prior the quartic model, because it needs less parameters. However, I think that are good reasons to keep symmetry restrictions for both the quartic and elipsoid models (see above). But there may be reasons in certain, and probably few cases, where the symmetry restrictions can be lifted, e.g. when you have direction dependent line shapes of the broadening contribution to the peak shapes. But maybe that could also be better modelled by direction dependent shape factors compatible with the crystal symmetry. A
Re: GSAS informations
>... to answer to your (too) long questions. May be later, OK? Going back to this quartics versus ellipsoids peak broadening stuff, maybe I can summarise: Why should the distribution of lattice parameters (=strain) in a sample match the crystallographic symmetry? If the sample has random, isolated defects then I see it, but if the strains are induced (eg: by grinding) then I'd expect the symmetry to be broken. Suggests to me the symmetry constraints should be optional, and that the peak shape function needs to know about the crystallographic space group and subgroups. Either the program or the user would need to recognise equivalent solutions when the symmetry is broken (like the "star of k" for magnetic structures). Why would anyone have anything against using an ellipsoid? That same function can be described by the quartic approach, it just has less degrees of freedom. In short, I don't understand why there is such a strong recommendation to use the quartics instead of ellipsoids or why the symmetry is not optional. I'm still persuing this because I have looked at something with a very small anisotropic broadening which seems to fit better with an ellipsoid which breaks the symmetry compared to a quartic which doesn't! Thanks for any advice, Jon
