Simulation of point defects in high-density luminescent crystals: Oxygen in barium fluoride

Abstract

Barium fluoride is an example of a high-density scintillator for detecting high-energy radiation. In use, its luminescent transmission is seriously degraded by radiation damage. This effect is associated with oxygen, among other impurities. At one time it was suspected that oxygen ${\mathrm{O}}^{\mathrm{\ensuremath{-}}},$ having been dissociated from a defect complex by radiation damage, absorbed some of the luminescent energy of the crystal. This explanation has now been abandoned, and the present work shows quantitatively that it is not correct. Specifically, a detailed study of the optical absorption of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ shows that its excitation energy, split by spin polarization, is $\ensuremath{\sim}50%$ higher than the luminescent frequencies of the crystal. Instead, color centers, such as $F$ centers, have come to be suspected. One origin of the color center is shown here to be the dissociation of a defect complex made up of an ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion bound to a fluoride vacancy, accompanied by electron transfer from oxygen to vacancy, forming an $F$ center. The study of the optical excitation of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ is used to assess the qualitative and quantitative importance of the main elements of the physical model and computational method in such a simulation. These elements include the ion-size effect of ${\mathrm{Ba}}^{2+}$ ions, spin-polarization effects in ground and excited states, electric quadrupole moment consistency between the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion and the embedding ${\mathrm{BaF}}_{2}$ crystal, basis set augmentation and optimization in the treatment of a quantum molecular cluster that includes the impurity for both ground and excited states, correlation correction, and projection of excited states onto spin eigenstates.