I am trying to change the isotropic thermal parameter to anisotropic and I cannot find a the right way to change the syntaxes in INP file to refine it anisotropically. I will appreciate your help
The main question is: why do you want to do this? The anisotropic description of thermal vibrations are a result of single crystal diffraction since ther you can see them... It has some reason that this is only possible for single crystal diffractometry. For powder software you can use as input file the anisotropic parameters but as far as I know, they will be internally immediately transformed into isotropic. The developer only want prevent that you do not miscalculate correct data.
Sometimes, a calculation does not make any sense (as in your case), and more often not everything makes sense if it is possible (the consideration of thermal parameters). First, they are often erroneous since they are "used" to compensate errors in structure description in order to reduce the R-values to smaller values (at any price), and second, many phase are described without any Debye-Waller factors. They certainly "vibrate" but it was not necessary...or better...the amount was not reliably detectable.
I only can recommend you to ignore these factors in general. Only if there is a reliable reason that it might be explain a certain behavior you should do this. Only because a software enables something does not mean that you should activate this option. Start with the parameters you understand and don't play with effects you have no experience. It will ALWAYS reduce the quality of your results, even if some reliability data decrease.
In Topas, you replace the beq keyword with adps. The kernel will then automatically put in the correct anisotropic parameters for your symmetry.
Whether or not your data supports their refinement is another question.
Thanks for all the answers, @Gertz the pyrochlore structure is known for its flexibility and disorder, and since I saw a common trend of underfitting specific peaks and overfitting another specific peaks, even in different compounds with the same structure, I thought I may need to consider the Beq as a Matrix, otherwise I will have very peculiar preferred orientations such 531. But I thanks for letting me know that anisotropic parameter can be refined for single crystal x-ray diffraction. To be honest, when I tried it, I got the matrix that did not make any sense.
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