Self-trapped excitons in SrFCl studied by ab initio methods.

Abstract

Self-trapped triplet excitons (STE's) on the ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ or ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ sublattices of SrFCl have been studied using quantum clusters embedded within point ions or self-consistently within shell-model ions. The unrestricted Hartree-Fock method of calculation is used to treat the clusters. Three possible STE's are studied. When the in-plane [100] ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ STE is displaced from an on-center position the total energy decreases and it becomes basically a one-center species with a hole on one ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ ion and the excited electron on the incipient ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ vacancy. The Cl-Cl bond length becomes 3.46 \AA{}, reflecting the one-center character of this species. The ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ out-of-plane STE behaves similarly with displacement in the [111] direction. The ${\mathrm{F}}_{2}^{\mathrm{\ensuremath{-}}}$ STE remains as a molecular species with displacement in the [110] direction. The hole becomes localized on the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ion nearest the incipient ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ vacancy and on which the excited electron becomes localized. There is a large barrier for electron-hole separation for all of these STE's. The presence of an adjacent halide vacancy allows the electron and hole to separate into a ${\mathit{V}}_{\mathit{K}}$ center and an F center without going through an intermediate H center, as occurs in alkali halides.