Evidence of Undistorted 8-Coordinated Cubic (Oh) Ce3+ Center in Moderately-Doped (0.01 mol %) CaF2 Single Crystal

The complex absorption bands of a cerium (Ce)-doped calcium fluoride (CaF2) single crystal has long been interpreted in terms of crystal field splitting and presence of plurality of Ce centers with reduced site symmetries. To substantiate this interpretation, the crystalline environment of the Ce ion in a moderately doped (0.01 mol %) CaF2 single crystal is evaluated using Ce K-edge X-ray absorption spectroscopy (XAS) at room temperature. The Ce-doped CaF2 crystal investigated exhibits multiple vacuum ultraviolet (VUV) absorption bands from 180 to 300 nm and two UV emission peaks around 320 and 340 nm. These UV emission bands have a light yield of up to 66.9% under 300-nm excitation as well as fast decay times of 40.26 ns (320 nm) and 38.38 ns (340 nm). Through our XAS analysis, we have confirmed that the Ce ion substitutes to the crystal's Ca2+ site with an oxidation state of 3+. However, our evaluation of the Ce3+ ion's crystalline environment reveals that Ce3+ maintains a cubic symmetry, and no local distortion is induced by the substitution.