Dear Mr Werner, adding to the remarks by Bob: basically the squared line widths (Gamma^2 in the GSAS manual) must not become negative for any hkl combination. For h00, 0k0, 00l, negative S400, S040, S004 always will lead to negative squared widths. If S400, S040 and S004 are all positive, the squared width can nevertheless become negative, if certain S220 etc. become "too negative", and in case of simple symmetry one may identify limits in comparsion with S400, S040 and S004. In some programes it is possible to impose restrictions and limits (e.g. Topas), which is however (to my knowledge) difficult in GSAS. The actual physical background for the limits for SHKL is the fact that the "Stephens model" is based on a correlated multivariate lattice parameter distribution, and the correlation coefficients between the lattice parameters must fall between -1 and 1.
Best regards Andreas Leineweber Dear All My questions concerns Stephen's anisotropic microstrain parameters in GSAS' profile function no. 4. Are negative values of these parameters (Shkl)) acceptable, i.e. do they have any physical/chemical meaning? In case negative values are not "allowed", how should one proceed (e.g. fix the ill parameters to 0?)? Thanks for your advice. Franz Werner -- Psssst! Schon vom neuen GMX MultiMessenger gehört? Der kann`s mit allen: http://www.gmx.net/de/go/multimessenger?did=10
