Optical and selected thermal properties of samarium-doped fluorochlorozirconate (FCZ) glass-ceramics: Formation and growth of BaCl 2 nanocrystals in FCZ glass-ceramics

Abstract

Photoluminescence (PL) and conventional and modulated temperature differential scanning calorimetry (DSC and MDSC) experiments have been carried out on a typical fluorochlorozirconate glass and glass-ceramics doped with Sm 3+ and Sm 2+ introduced by the addition of SmF 3 and a reducing agent, NaBH 4, into the initial mixture of constituents. The nominal FCZ composition was 53% ZrF 4, 20% NaF, 3% AlF 3, 3% LaF 3, 1% SmF 3, 1% BaF 2, 19% BaCl 2 (molar percentages). Prior to DSC and PL measurements, some of the glasses have been heat treated (annealed at an elevated temperature) under different conditions, which has resulted in glass-ceramics containing BaCl 2 nanocrystals with a hexagonal and/or orthorhombic crystal structure, depending on the heat treatment conditions. Hexagonal nanocrystals may be obtained by a simple one step annealing process while the formation of orthorhombic crystallites requires sequential multistep annealing treatments. Long duration, low temperature annealing, required for the formation of orthorhombic BaCl 2 nanocrystals, leads to the appearance of an endothermic enthalpy peak at around 250 °C on the conventional DSC thermogram. Temperature modulated DSC experiments identify a clear glass transformation in this regions; and the endothermic peak has been attributed to the structural relaxation enthalpy in the host glass which is usually obscured by the thermal effects associated with the formation of BaCl 2 nanocrystals. The observed thermodynamic effects correlate with the suppression of the broad PL band around 900 nm, which is most likely due to Sm 2+ ions near or in the “shell-region” of the glass surrounding the nanocrystals.