It is indeed advisable to start with an internal standard to check that there is no instrumental error or sample position issue (sample displacement is the more common cause of peak shift). Silicon powder is often used as a standard, a standard reference material (SRM) is sold by the NIST (SRM640) but you can also use good quality silicon powder (99.99%) if you don't need an accuracy better than 0.0001A (a=5.4309A).
Rietveld programs are indeed not necessary if you only the lattice parameters, standard peak fitting and lattice parameter refinement using several peaks at high angle should give you good results. Fullprof can handle internal standard: you have to fix the lattice parameter of Si to its known value and refine zero error or sample displacement parameter (use Full profile fit prerentially as indicated by Leopoldo). Patrick ________________________________ De : Leopoldo Suescun <leopo...@fq.edu.uy> À : Gumelar Pritosiwi <pritos...@gmail.com>; rietveld_l@ill.fr Envoyé le : Jeudi, 14 Mai 2009, 15h45mn 16s Objet : RE: Introduction and Unit cell parameter determination Dear Gumelar Pritosiwi, There are two independent topics in your question. If you are using a well calibrated conventional diffractometer to determine your cell parameters then you don’t need an internal standard. That would be useful if you, additionally, need to do a quantitative phase analysis. In a conventional machine with sealed tube source the cell parameters determination can be done easily in the case of cubic magnetite, because you already know the incident wavelength and d-spacings for the observed maxima are linearly related to “a” cell parameter by d(hkl)=a/sqrt(h^2+k^2+l^2). Programs such as FULLPROF, GSAS, TOPAS, etc. are meant for Rietveld refinement, that is something a little bit more complex than cell parameters determination, although most of these programs have additional routines (peak search and indexing in Winplotr) that can help determine cell parameters of an unknown phase, or in your case, of the magnetite you are studying. You can also try full profile fit (LeBail or Pawley methods) to extract best possible cell parameters provided your data is of enough quality and you are willing to cope with profile shapes, background modeling, etc. (too complicated to start with in my opinion). If you use data with internal standard for lattice parameters determination you may choose to ignore the internal standard peaks or just collect new data on the raw material where you just see your magnetite peaks (be careful, if your sample contains other crystalline materials you will need to identify the extra peaks and avoid using them for the cell parameters determination). Free programs like checkcell will allow you to determine your cell parameters by refinement of previously determined peak positions, other programs like Winplotr, PowderX, etc will allow you to perform a peak search and extract a peak list, other programs like CMPR can fit individual peaks and output a list of fit results. Any of these (and many more) you can then fit to obtain “a” using the abovementioned equation. There are probably many more free programs to do either thing more or less automatically so you should check the www or ccp14 site for information. If your magnetite show any kind of structural distortion this may be a bit more complex, but in that case you’ll need to check some crystallography book to get a better idea of what can you do with your powder diffraction data and how can you do it. A good book for beginners that gets all the way up to complex stuff is “Fundamentals of Powder Diffraction and Structural Characterization of Materials” by V.K. Pecharsky and P.Y. Zavalij edited by Springer. Hope this helps. Leo Dr. Leopoldo Suescun Prof. Agr (Assoc. Prof.) de Física Tel: (+598 2) 9290648/9249859 Cryssmat-Lab./DETEMA Fax: (+598 2) 9241906 Facultad de Quimica, Universidad de la Republica ,_. | \ | v- ,' \ | ( \__Montevideo, Uruguay From:Gumelar Pritosiwi [mailto:pritos...@gmail.com] Sent: Thursday, May 14, 2009 8:30 AM To: rietveld_l@ill.fr Subject: Introduction and Unit cell parameter determination Dear all, I am a new member of this mailing list. My background is environmental engineer. At the moment I am conducting a research related to the crystalline iron oxide magnetite. One of the analyses that I have to do is the determination the unit cell dimension (lattice parameter) of the crystal from X-ray powder diffraction data. So far, for the XRD analysis I have mixed my sample with an internal standard. Because I am a beginner in the crystallography, I would like to hear opinions on two things: - Is using an internal standard a good way of sample preparation for the determination of unit cell?. Can anybody give me an example of a good internal standard that can be used for XRD analysis?. - Can anybody give me a suggestion which free software can be used for the determination of unit cell parameter from XRD powder diffraction data (with internal standard)?. I have learned a little about FullProf but I don?t know whether this program can handle XRD data with internal standard or not. My knowledge about FullProf is very limited, therefore if some hints about this program are more than welcome. Is perhaps FullProf too complicated for me as a beginner?. ? Thanks a lot and best regards ? Gumelar Pritosiwi -- Gumelar Pritosiwi Institute of Environmental Technology and Energy Economics Hamburg University of Technology Eissendorfer Str. 40 21073 Hamburg Tel ?+49 40 42878 3319 Fax ?+49 40 42878 2315 No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.325 / Virus Database: 270.12.29/2114 - Release Date: 05/14/09 06:28:00
