A computer simulation study of oxygen defect centers in BaFBr and BaFCl

Atomistic simulation techniques are used to examine several oxygen trapped-hole centers resulting from X-irradiation of BaFBr and BaFCl crystals. The calculations employ recently derived interatomic potentials for the oxide ion-host anion interactions. Particular attention is focussed on the sites occupied by the oxide impurity and the energetics of ionization. Our results show the defect model involving O- substitutional at a Br-/Cl- site to be a favorable trapped-hole center, in accord with the assignment proposed from electron paramagnetic resonance measurements. The defect simulations find a large energy barrier to oxide interstitial formation from the oxide precursor at a substitutional site, which suggests that conversion from Ox- to Oint- is highly unlikely as has been demonstrated in EPR experiments. Ion displacements following lattice relaxation about the defect are also examined. The position of O- substituted for Br- in BaFBr is computed to be displaced from the regular lattice site by 0.53 A, along the c axis towards the Ba2+ ion plane, in agreement with models derived later from ENDOR experiments, while O- substituted for F- remains on the lattice site in agreement with experiment. (author)