Charge-State Stability and Optical Transitions of Oxygen Impurities in Barium Fluoride Crystal
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ABSTRACT The linear combination of atomic orbital-molecular orbital embedded-cluster is used to investigate the electronic structure and binding energies of small oxygen impurity clusters in the local density theory. Binding energies for substitutional oxygen ions in BaF2 crystal are studied for its 2-, 1-, and 0 charge states. These are used to assess charge-state stability. The energy eigenvalue spectra and optical transitions are obtained by transition state calculations for the oxygen impurities. These results of optical transition calculations obtained are very good and provide a useful interpretation of the ultraviolet-absorption experiments. INTRODUCTION BaF 2 is one of the most radiation resistant fast scintillating crystal known [?]. Systematic studies of the radiation damage phenomena of BaF 2 have been performed . Tests show [?] that the radiation damage of BaF 2 after either -y - ray or neutron irradiation caused by various type of impurities or defects, not by the material of interest. It is likely that certain anion impurities such as oxygen are critical in causing radiation damage. However, oxygen is especially difficult to defect in fluoride crystals due its chemical similarity to fluorine. It has been known for several years that the alkaline earth fluorides may easily become contarninated with oxygen and hydroxyl. Information on the structure of the defects formed has been obtained from electron spin resonance studies 1?7.However, the electronic structure has not been established for the majority of the oxygen containing centers. Oxygen containing impurities evidently, participate in the increase of absorption bands in the ultraviolet region. Although brief studies of Nakata R. (?] have suggested that 02- ions may play an important role, the nature of oxygen impurities has not been elucidated and the charge state of oxygen impurities is uncertain, as it is unknown whether isolated oxygen ion exists in the alkaline earth fluoride crystals in the 1or 2- charge state and which ion is responsible to the ultraviolet absorption. Some experiments
indicate that the crystals containing trace amounts of oxygen impurities in the region of several ppm level, show significantly radiation damage. To understand the nature of oxygen impurities in host BaF 2 , theoretical studies of the electronic structure and charge state stability and calculations of the experimentally observed characteristics of impurity defects beginning with the simple models, have been performed. A theoretical study of the charge-state stability and electronic structure of Oc, 0- and 02centers in BaF 2 and the explanation of the nature of optical absorption and luminescence of the oxygen impurity centers in BaF 2 crystals is the goal of the present paper. We did fully selfconsistent local-density calculations of the energy spectrum, Mulliken population and binding energy of oxygen impurity clusters.
METHOD As described in previous works,we use the self-consistent-field local-density formalism in a molecular-orbital (MO) framework to descr
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