RE: Size Strain in GSAS

[Von Dreele, Robert B.]

Title: Message

Nic, I know about "super Lorentzians". Trouble is that many of those older reports were from Rietveld refinements "pre TCH" and used a formulation of the pseudo-Voigt which forced the FWHM of the Gaussian and Lorentzian components to be equal and allowed the mixing coefficient (eta) to be a free variable (n.b. it is not free in the TCH formulation). Thus, these ought to be discounted in any discussion about the occurence of super Lorentzian effects in real samples. Bob     R.B. Von Dreele IPNS Division Argonne National Laboratory Argonne, IL 60439-4814   -----Original Message----- From: Nicolae Popa [mailto:[EMAIL PROTECTED] Sent: Thursday, April 14, 2005 8:10 AM To: rietveld_l@ill.fr Subject: Re: Size Strain in GSAS   Right, is rare, but we have meet once. A cerium oxide sample from a commercial company, c=2.8. I don't know if they did deliberately, probably not, otherwise the hard work to obtain such curiosity is costly and the company risks a bankruptcy. On the other hand superlorenzian profiles were reported from a long time, only were interpreted as coming from bimodal size distributions. And third, you see, people have difficulties to extract size distribution from the Rietveld codes as they are at this moment.   Nicolae Popa   A word from a "provider" of a Rietveld code (please don't call me a "programmer"). "But if c>0.4 any pV fails" - OK, for what fraction of the universe of "real world" samples is "c">0.4? I suspect, given the general success of the TCH pseudoVoigt function, that it is exceedingly small and only occurs when one works hard to deliberately make a sample like that.     R.B. Von Dreele IPNS Division Argonne National Laboratory Argonne, IL 60439-4814   -----Original Message----- From: Nicolae Popa [mailto:[EMAIL PROTECTED] Sent: Thursday, April 14, 2005 7:14 AM To: [EMAIL PROTECTED] Cc: rietveld_l@ill.fr Subject: Re: Size Strain in GSAS >Dear Nicolae, >Maybe ya ploho chitayu i ploho soobrazhayu, but even after your >explanation I can't see a way to calculate <R> from the results of >fitting described in chapters 6 & 7 of JAC 35 (2002) 338-346. From such >fitting you obtain only dispersion parameter c. Or I missed something? >Anyway, being "Rietvelders" we still have to deal with TCH-pV function >and we need to extract as much as possible correct information from it. >Hope we shall see more appropriate functions for microstructure >analysis in popular Rietveld programs. >Cheers, >Leonid Dear Leonid,   Indeed you missed something. I presume you have the paper. Then, take a look to the formula (15a). This is the size profile for lognormal. There is the function PHI - bar of argument 2*pi*s*<R>.  Replace this function PHI - bar from (15a) by the _expression_ (21a) with the argument x=2*pi*s*<R>. You get it? So, not only "c" but also <R>.   "We are Rietvelders" means that we must be only "codes drivers", "cheffeurs des codes", "voditeli program"? Have we to accept the "Procust bed" of the Rietveld codes at a given moment? All Rietveld codes are improving in time, isn't it?   In particular for the Round_Robin sample TCH-pV works because c=0.18. (Davor explained how Dv and Da are found). But if c>0.4 any pV fails.   Best wishes,   Nicolae