A study of two electron defect systems in alkali halide crystals.

Abstract

The structures of two electron defect systems in alkali halide crystals are studied. The systems which have been studied include: two electrons localized at an anion vacancy (F$\sp\prime$-centre); positronium self-trapped at an anion vacancy (Fe$\sp+$-centre); positron self-trapped at a cation vacancy (F$\sb{\rm anti}$-centre); positronium self-trapped at an interstice; and positronium in a Bloch state. An improved version of the extended-ion method which is based on the one electron Hartree-Fock approximation is used to perform these calculations. Its main feature is the exclusive use of floating 1s Gaussian functions as basis. For the multi-electron defect systems, the calculation of matrix elements of two electron interaction terms is a most difficult problem. We developed an effective approach to treat this interaction approximately. The correlation effect of defect electrons is partly accounted for by properly arranged Gaussian basis. The binding energy, thermal dissociation energy, and transition energy between ground state and excited state are calculated for F$\sp\prime$-centres. A defect model with negative-U properties was introduced to interpret the deeply bound F$\sp\prime$-centre. Calculations of positron binding energies are made for Fe$\sp+$-centres and F$\sb{\rm anti}$-centres. In addition, we evaluate the angular correlation and lifetime of an annihilated electron-positron pair for Fe$\sp+$-centres, localized positronium and Bloch state positronium. The observed phenomena such as the transition of positronium from Bloch state to localized state, and the crystallographic effect are examined theoretically. The calculated results regarding various properties of crystals are in reasonably good agreement with experiment.