Dear colleagues, I use Rietveld method for quantitative phase analysis. I use Topas 5 and now I have started using Spherical Harmonics for correction of preferred orientation (instead of March-Dollas correction). I mainly collect data in Bragg-Brentano geometry; samples are usually prepared by front or side-loading. My question is: Which order of Spherical Harmonics do you recommend to use for the purpose of QPA? i.e. for mineral phases like calcite, mica, feldspars, plagioclase, kaolinite? (Most atomic position are usually fixed, refined are only scale factors, unit-cell parameters, size, Debye-Waller parameters…). I know that using high order Spherical Harmonic may lead to nice fits, however with meaningless results and therefore the high orders should be used with cautions. I observe the drop of Rwp when introducing the Spherical Harmonics correction and when it becomes insignificant, than I reduce the order. E.g. for calcite, it is order of 2, for kaolinite order of 4 (i.e. using higher order lead only to insignificant drop of Rwp). Your suggestions, experiences and comments are welcome. Frantisek Laufek Czech Geological Survey, Prague Czech Republic
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