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 °C produces small (∼6–11 nm) hexagonal BaCl₂ crystallites, while temperatures in the range 270–290 °C lead to the formation and growth of larger (∼15–100 nm) orthorhombic BaCl₂ crystallites, as well as additional unidentified phases. We observe only weak PSL from glass ceramics containing hexagonal BaCl₂ (∼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 ∼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 ∼7 nm surrounding the BaCl₂ crystallites. [ABSTRACT FROM AUTHOR]