> Dear Rietvelders
> I am refining a structure of natural andradite-Ca3(Fe,Al)2Si3O12 and
> encounter some difficulty. The isothermal parameter of Ca-atom is very high
> (2.9) whereas the others are all negative. I check the positions of all
> atoms are correct; and the R-factors are good. I wonder what causes this
> strange behaviour of Ca- the instrument problem or its own andradite
> structure? I tried to replace some of Ca-atoms with another possible cations
> but did not help much.
> Could anybody help me to understand this phenomenon?
> Thank you very much.
> Watchara
>
Try the following: constrain all isothermal parameters as one refinement
parameter
or fix them at reasonable value, and refine occupational factor for Ca. Usually
high temp. factor for some atom means that the atom is either not fully occupied
or
mixed statistically with other lighter element...
--Dr. Peter Y. Zavalij University Crystallographer Materials Research Center, SUNY at BinghamtonPh/Fax:(607)777-4623 E-mail:zavalij@binghamton.eduhttp://imr.chem.binghamton.edu/zavalij/zavalij.html