Optical properties of divalent samarium-doped fluorochlorozirconate glasses and glass ceramics

Glass ceramics comprising Sm2+-doped barium chloride in both hexagonal and orthorhombic phases embedded in a fluorochlorozirconate glass matrix have been prepared. Divalent samarium doping was effected with the aid of NaBH4 to partly reduce Sm3+ in the starting materials to Sm2+. The optical absorption spectrum in unannealed samples shows a strong, broad 4f → 5d band centred at around 340 nm and attributed to Sm2+, but no photoluminescence is observed. Following annealing to induce crystallisation of Sm2+-doped barium chloride crystals in the glass, photoluminescence is observed in the form of sharp line 4f → 4f transitions, and two broad 5d → 4f bands, the latter centred at around 680 and 900 nm. The 680 nm band disappears below about 250 K whilst the 900 nm band intensity increases with decreasing temperature. The 4f → 4f transitions for Sm2+ ions in the orthorhombic phase of barium chloride show an abrupt change in the relative intensity of transitions originating from the 5D1 and 5D0 levels at around 90 K. The effect is attributed to the role of the 5d level in thermally assisted indirect transitions between the 5D1 and 5D0 levels. In contrast, transitions from only the 5D0 level are observed for the hexagonal phase. The temperature dependence of the photoluminescence intensity, and the observation of a different crystal field splitting pattern for the 7F1 level, can be used to distinguish between the two phases. The splitting pattern for the 7F1 level for the hexagonal phase of barium chloride suggests that both possible crystallographic barium sites are occupied by Sm2+ ions.