Just to add more fat to the fire.... Have a look at Young, R. A. & Desai, P. 1989, 'Crystallite Size and Microstrain Indicators in Rietveld Refinement', Archiwum Nauki o Materialach, vol. 10, no. 1-2, pp. 71-90. (I can send the PDF if needs be) They talk about the Thompson, Cox and Hastings model, which explicitly separates the gaussian and lorentzian components of a psuedo-Voight peak shape. FWHM(G)^2 = U tan^2(T) + V tan(T) + W FWHM(L) = X tan(T) + Y/cos(T) As Prof. Stephens pointed out (and is stated in Yound and Desai), the coefficients can be broken into instrumental and sample (size, strain) components. U = U_inst + U_strain V = V_inst W = W_inst X = X_inst + X_strain Y = Y_inst + Y_size You can fix the instrument components with your standard, and then refine the difference with your sample. If you want to stick with the straight UVW symbolism, Young and Desai also state that you can use the size broadening term FHWM(G)^2 = Z/cos^2(T), which yields: FWHM(G)^2 = Z/cos^2(T) + (U_inst + U_strain) tan^2(T) + V_inst tan(T) + W_inst which can be re-written as FWHM(G)^2 = (U_inst + U_strain + Z_size) tan^2(T) + V_inst tan(T) + (W_inst + Z_size) as long as you constrain the two Z_size's to be the same. The last equation is what Prof Stevens alludes to in his "refinement of U and W", all of the sample related parameters are folded up there. Of course, your mileage may vary...
Cheers Matthew ________________ Matthew Rowles CSIRO Minerals - Clayton Ph: +61 3 9545 8892 Fax: +61 3 9562 8919 (site) Email: [EMAIL PROTECTED]
