Photostimulated luminescence from a fluorobromozirconate glass-ceramic and the effect of crystallite size and phase

We report a systematic study of the photoluminescence (PL), photostimulated luminescence (PSL) and thermostimulated luminescence (TSL) from europium- and bromine-doped fluorozirconate glas-ceramics. Eu2+, ions in the as-prepared glass show no PL, but after suitable thermal annealing hexagonal phase and orthorhombic phase barium bromide crystallites are precipitated and PL is observed from Eu2+ ions in these crystallites. Room temperature PSL is observed from the orthorhombic phase, with an efficiency which is up to Ob of the well known crystalline storage phosphor BaFBr:Eu2+. The emission is at 404 nm, and there is a maximum in the stimulation at 580 nm. We associate the PSL with an optically quenchable peak in the glow curve, which has an activation energy of 1.20 eV and attribute this feature to a perturbed F centre. Room temperature PSL from glass-ceramics containing predominantly the hexagonal phase of BaBr2 has a relative efficiency of less than 0.07%. The resultant trap depth determined from the glow curve is 0.60 eV, which suggests that this trap does not arise from a perturbed F centre. We find that the PSL efficiency systematically decreases with decreasing hexagonal BaBr2 nanocrystallite size and there is no detectable PSL from nanocrystallites smaller than 10 nm. However, below 100 K, the PSL efficiency for the hexagonal phase is nearly as high as for the orthorhombic phase and the concomitant electron traps are perturbed F centres. The measured low temperature emission was at 409 nm and an additional stimulation peak was observed at 700 nm. The PSL in this case is thermally bleached above about 150 K.