On Wednesday 28 May 2008 18:39:40 益豪 wrote:

We have complex perovskites, the series of La(2-x)/3 Nax Mg0.5 W0.5 O3 where x=0,0.1,0.2,0.3,0.4,0.5. I would like to fit the EXAFS data, and according as ICSD to build the atom list by using TkAtoms. However these samples might have phase transformation with the increase in x-value. First, I use the same structure which the x=0 atom coordinates from ICSD to fit the data of x=0-0.2, and use the x=0.5 atom coordinates to fit the data of x=0.3-0.5. But the trend of the distance between the central atom(W) and the six neighboring oxygen atoms with Na concentration is very strange! The distance of O-W with x=0-0.2 is increasing, x=0.2-0.3 is decreasing, and x=0.3-0.5 is increasing. I conjecture that the result is so strange because we use the different space group. I would like to ask how to choose the atom coordinates x=0 and x=0.5 from ICSD to build the theoretical model of x=0.1-0.4.

Hi! Your question is not completely clear. As I understand it, when x=0 or x=0.5 you are able to use the crystal data from the ICSD as the basis of your analysis of the W edge data and obtain a good result in your exafs analysis. However, for the intermediate values of x, you find peculiar values for W-O distances. I would expect that, in the locations occupied by Na, there is significant local disortion since Na is (I presume) smaller than La. Consequently, the oxygen atoms might relax around the Na. In that case, you might see some significant distortions to the oxygen octahedra which will show up in your exafs data as additional disorder in the W-O distribution. It is common to presume that by "phase transition" you mean crystallographic phase transition. Have you done diffraction on the materials with intermediate values of x? Do you observe those materials to be in a different phase from the end members? If so, and if no no one has ever published crystallography on those phases, perhaps you need to refine that diffraction data yourself. And if you do not observe a crystallographic phase change, then probably you are seeing the effect of local disorder. To interpret your exafs data you will need to somehow model that disorder. I hasten to point out that everything I have said is speculation. I have never looked at materials quite like yours and I have no personal knowledge about your system. You should read the papers on mixed ferrites by Scott Calvin or my paper of BaTaO2N to see how similar problems have been addressed by the so-called experts on this list. Hope that helps, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/