Structural phase changes in barium bromide nano-crystals in a fluorobromozirconate glass-ceramic x-ray storage phosphor

The structural changes in a fluorobromozirconate glass ceramic containing a ratio of 5% bromine to fluorine ions, following thermal annealing in the range 240-300 °C, are reported. The changes were monitored through x-ray diffraction, and the photoluminescence (PL) and electron paramagnetic resonance of Eu 2+ dopant ions. In the range of 240-275 °C, the barium and bromine ions in the glass precipitate to form the metastable hexagonal phase of barium bromide. The Eu 2+ PL spectrum comprises a narrow band at 410 nm, and a weaker broad band centred at 485 nm. The 410 nm band is assigned to two unresolved 4 f 6 5 d 1 →4 f 7 emissions from Eu 2+ ions at the two Ba 2+ sites in this phase, whilst the 485 nm band is assigned to an impurity associated site or cluster. On annealing at 290 °C, the hexagonal phase transforms to the stable orthorhombic phase, and the PL spectrum comprises a single narrow band centred at 404 nm, assigned to a 4 f 6 5 d 1 →4 f 7 emission from Eu 2+ ions at the single Ba 2+ site in the orthorhombic phase.