Re: Size Strain in GSAS

[Nicolae Popa]

Title: Message

Alan,   (i) but a sum of two Lorentzians is not sharper than the sum of two pVs (Voigts)?   (ii) We  fitted the exact size profile caused by the lognormal distribution by a pV (for low lognormal dispersion) or by a sum of maximum 3 Lorenzians (for large lognormal dispersion). This is "cheaper"  than the sum of 2 pVs. It involves the calculation of maximum 3 elementary functions with 4 independent parameters (3 breadths + 2 mixing parameters minus 1 constraint = 4)  Sum of two pVs presumes 4 elementary function and 5  independent parameters (2 for one pV + 2 for the second one + a mixing parameter).   Best wishes, Nicolae       A pure peak fitting approach shows that two pV�s (or two Voigts) when added with different FWHMs and integrated intensities but similar peak positions and eta values can almost exactly fit Pearsons II functions that are sharper that Lorentzians. This is not surprising as both profiles comprise 6 parameters.   Thus from my observations two pVs added together can fit a bimodal distributions quite easily. In fact my guess is that two pVs can fit a large range of crystallite size distributions.     all the best alan