Photostimulated luminescence from fluorochlorozirconate glass ceramics and the effect of crystallite size

We report the results from a study of europium-doped fluorochlorozirconate glasses that have been thermally processed to induce nucleation and crystallization of BaCl2 crystallites. The resulting glass ceramics show a photostimulated luminescence (PSL) effect with a conversion efficiency which is up to 80% of that found in the commercial crystalline x-ray storage phosphor material BaFBr:Eu2+. Thermal processing for 20 min in the range 240-260 degrees C produces small (similar to 6-11 nm) hexagonal BaCl2 crystallites, while temperatures in the range 270-290 degrees C lead to the formation and growth of larger (similar to 15-100 nm) orthorhombic BaCl2 crystallites, as well as additional unidentified phases. We observe only weak PSL from glass ceramics containing hexagonal BaCl2 (similar to 0.1% conversion efficiency compared to BaFBr:Eu2+ at room temperature), but orthorhombic phase crystallites give rise to a much larger relative efficiency that increases with particle size, and reaches similar to 80% for 100 nm diameter particles. The PSL is attributed to the excitation of electrons trapped as F centers in the BaCl2 crystallites, while the dependence on crystallite size is consistent with a PSL-inert surface shell of thickness similar to 7 nm surrounding the BaCl2 crystallites. (C) 2005 American Institute of Physics.