GENERATION OF F-CENTERS AND HOLE-CENTERS IN THE NONSTOICHIOMETRIC X-RAY STORAGE PHOSPHOR BAFBR

After x-irradiation of nonstoichiometric BaF1.1Br0.9, besides the F(Br -) centre a new intrinsic hole centre was found by electron paramagnet ic resonance (EPR). The powder EPR line of this new centre can be simu lated assuming an F; molecule on a fluoride site. The new hole centre is stable at room temperature and is the anti-centre of the F(Br-) cen tre in the F centre generation process. Magic angle spinning nuclear m agnetic resonance (MAS-NMR) experiments on nonstoichiometric BaF1.1Br0 .9 show not only resonances of F-19 on regular lattice sites, but also resonances which are due to F-19 on bromine sites ('fluorine antisite s'). There are about 10% fluorine antisites in the nonstoichiometric p owder which corresponds to the excess fluorine determined by chemical analysis. The generation of F(Br-) electron trap-hole trap pairs upon x-irradiation originates at the fluorine antisites.