Hey all, I've been having a poke around with GSAS-II, testing it out by using it to do some fairly basic multiphase quantitative analysis, and I'm having a bit of trouble calculating the amorphous content. I'm using the method as outlined in the paper 'Rietveld quantitative amorphous content analysis' by A. G. De La Torre et al. in which the overestimation of the internal standard is used as the basis for estimating the amorphous content. I'm using the phase fraction/weight fraction of the standard from the .lst file for input into the equation described in that paper.
The problem I'm having is that despite getting a relatively good fit to the pattern (down to Rwp of approximately 4% or so), the Rietveld weight percent of the internal standard is coming out lower than the actual (weighed) weight percent that was added to the sample. Which results in a negative percent amorphous content. So I'm just wondering if there's any known issues with calculating amorphous content this way, or if there's a preferred method amongst GSAS users that might yield better results. As a side note I've previously used TOPAS to calculate the amorphous content on a set of data, and I've tried using that data as a control to see what GSAS-II would come out with. That data showed up as 50% amorphous content according to TOPAS. Whereas GSAS-II showed around 5%. Of course there are probably differences in how weight percent & amorphous content are calculated between the programs but the size of that difference is a contributing factor to my worry that I've missed something fundamental. In terms of additional information, I'm working with laboratory XRD data with cobalt radiation, with corundum as the internal standard. If there's any other info that would help with advice please let me know and I'll do my best to provide it. Anyway, sorry for the wall of text, hope this isn't too trivial a question to be posting here. :) Kind regards, Phill -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mailman.aps.anl.gov/pipermail/gsas-ii/attachments/20130731/f4f863be/attachment.html>
