> It was shown in paragraph 6 of JAC 35 (2002) 338-346 that > size-broadened > profiles given by both lognormal and gamma distributions can be > approximated > by a weighted sum of Lorentz and Gauss functions for a broad range of > distribution dispersions. Besides, round robins can sometimes be long > adventures...
Yes, profiles can be approximated, but the question is not in approximating profiles. The primary topic of the discussion is "Size Strain in GSAS". GSAS and most other Rietveld refinement programs use TCH-pV profile function which provides the simplest and more or less correct way for separating microstructural and instrumental broadening contributions. Unfortunately, the microstructural parameters such as Dv and Da sizes derived (classically) from TCH-pV deviate significantly from reality for narrow and broad dispersions. That's why the TCH-pV-based calculations of Dv, Da or average crystallite diameter need to be modified and calibrated on, at least, simulated data for various dispersions. The formalisms presented in chapters 6 and 7 of JAC 35 (2002) 338-346 are more complex and they don't give a clear way for separating microstructural from instrumental effects and, besides, for estimating the values of Dv, Da or <R>. Leonid __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
