Density Functional Calculations of F-Center Related Positron Annihilation in Alkali Halides.

Positron annihilation is useful as a probe in the study of electronic structure in a wide range of condensed matter systems. Positron lifetimes often hold key information about the presence of defect electronic states. Although the experimental data may contain information about the presence of defects in solids, the analysis of that information in terms of local defect structure may be difficult. Many of these problems of data analysis could be resolved with theoretical calculations of the expected positron lifetimes for the bulk materials and associated defects. Until now, most of theoretical calculations of the positron lifetimes in defect crystals have been based on parameterized fits to experimental data. In systems such as these where the experimental characterization is incomplete or uncertain, parameter-free tools are essential. A first-principles method for the self-consistent calculation of the positron states in many electron systems has been developed, but some important features have not been fully implemented. In this work, density functional calculations are carried out for positron bound states by using the self-interaction corrected, local spin density approximation of density functional theory and by using the linear combinations of atomic Gaussian orbitals to the point cluster in the embedded-cluster method for crystals. Positron lifetimes are calculated for the halide and alkali atomic systems rm (H^-{:}e^+, F ^-{:}e^+, Cl^-{:}e ^+, Li{:}e^+, and rm Na{:}e^+) for several bound states. These calculations provide the reference for the lifetime shift by including crystal field in alkali halides (LiF, NaF, LiCl, and NaCl). Finally, the well -documented crystal system can be applied into the positron bound states in defect centers rm (F_ {c}{:}e^+) in the alkali halides listed above, and the local features of the positron bound states can be analyzed from relationships between the annihilation rates and the lattice constants for alkali fluorides (LiF, NaF) and the corresponding alkali chlorides (LiCl, NaCl).