Negative Biso

[Hongwu Xu]

Dear All,

I'm currently doing refinement for a Cs-Zr-Si-O phase using GSAS. While
the refinement proceeded stably and the final cycle got converged, the
displacement factors (Biso) were negative except for Cs. I tried with two
sets of data collected on two diffractometers, and the situations are the
same. Could anyone out there expain to me what this means? Any suggestions
to solve this problem?

Many thanks,
Hongwu