Cameron MacKeen, Frank Bridges, Apurva Mehta, Michael Kozina

University of California, Santa Cruz

Materials similar to SrF₂  and CaF₂ doped with rare earths  are known for their applications in lasers, use as phosphors for lighting, and optical amplification, but they exhibit unusual new behavior at low dopant concentration (~.01%). Upon exposing as-made CaF₂:Yb.01%   to x-rays at the Yb LIII edge near 200K, the Yb ion experiences a substantial valence reduction; little change happens at room temperature or at 10K. This reduced state is relatively stable at low temperatures, and reversible upon heating the sample to room temperature. One model suggests this excited reduced state is a consequence of changes in the charge-compensating interstitial fluorine atoms. We report on the change in the x-ray absorption near edge structure (XANES) of this material before and after x-ray exposure. In addition,  by using x-ray absorption fine structure (EXAFS), we find a shift in Yb-F bond length corresponding to the change in ionic radius between Yb 2+ and 3+. Further study is needed to characterize time dependence and explain this anomalous excited state.