X-ray Excitation Triggers Ytterbium Anomalous Emission in CaF2:Yb but Not in SrF2:Yb.

Materials that luminesce after excitation with ionizing radiation are extensively applied in physics, medicine, security, and industry. Lanthanide dopants are known to trigger crystal scintillation through their fast d-f emissions; the same is true for other important applications as lasers or phosphors for lighting. However, this ability can be seriously compromised by unwanted anomalous emissions often found with the most common lanthanide activators. We report high-resolution X-ray-excited optical (IR to UV) luminescence spectra of CaF2:Yb and SrF2:Yb samples excited at 8949 eV and 80 K. Ionizing radiation excites the known anomalous emission of ytterbium in the CaF2 host but not in the SrF2 host. Wave function-based ab initio calculations of host-to-dopant electron transfer and Yb2+/Yb3+ intervalence charge transfer explain the difference. The model also explains the lack of anomalous emission in Yb-doped SrF2 excited by VUV radiation.

The determination of dopant ion valence distributions in insulating crystals using XANES measurements.

Ytterbium-doped wide-bandgap fluoride crystals CaF2, SrF2 and NaMgF3 have been measured using x-ray absorption near edge structure (XANES) on the L3 edge to determine the ratio of trivalent to divalent Yb ions present in the crystals. This study improves upon previous XANES measurements of dopant ion valency by taking into account the x-ray emission transition probabilities for the divalent and trivalent species instead of simply assuming that the relative concentrations may be determined by the ratio of the x-ray excitation band areas. Trivalent to divalent ratios as high as 5 are inferred even at low total dopant ion concentrations of 0.05 mol% Yb.