Preparation-induced F-centre transformation in BaFBr: Eu2+

In this paper the transformation of one type of photostimulable colour centre (F-centre) into another, depending on the preparation conditions of the 21 storage phosphor BaFBr:Eu2+, is investigated. Despite the fact that the quasi-binary phase diagram of BaF2-BaBr2 shows no detectable solid solution for either BaF2 or BaBr2 in BaFBr, it will be demonstrated that an imbalance induced on purpose in the bromine/fluorine ratio can alter the shape of the stimulation spectrum from F(F-)- to F(Br-)-centres and vice versa in a subsequent annealing process. It will be demonstrated that such an imbalance can be generated by exposing BaFBr:Eu2+ to an alcohol/water solution between a sintering step and an annealing step. Thereby the water dissolves BaFBr by removing BaBr2 from the surface of the grains, leaving an excess of BaF2 behind. Based on these findings a practical synthesis route will be offered, which can lead to improved phosphor properties. Such properties are a macroscopically single phase of BaFBr:Eu2+ without detectable BaF2 or BaBr2 residuals and solely photostimulable F(Br-)-centres.