Self-trapped hole (VKcenter) in NaCl-type alkali halides. Lattice relaxation and optical properties forMX:X2 −systems

The equilibrium lattice configurations, electronic excitation energies, and systematic exploration of the $V_K$ center in the 16 NaCl-type alkali halides, $\mathrm{MX}$ ($M=\mathrm{L}\mathrm{i},\mathrm{N}\mathrm{a},\mathrm{K},\mathrm{R}\mathrm{b}$; $X=\mathrm{F},\mathrm{C}\mathrm{l},\mathrm{B}\mathrm{r},\mathrm{I}$) are calculated. The defect is regarded as a $X_2^{ -}$ molecular ion with the electronic structure perturbed by the ionic environment, but with the equilibrium bond length only slightly modified. The energy required to form a $V_K$ center varies from ~ 4 eV (RbI) to ~ 7 eV (LiF), relatively large defect energies. The important relaxed ion positions are summarized, analyzed, and compared with experiment (for LiF and NaF). The $V_K \left(X_2^{ -}\right)$ electronic transition energies, $\Delta E_{\sigma }$, $\Delta E_{\pi }$, and $\Delta E_g$, are predicted from theory and compared to experiment. There are clearly discerned trends in transition energies with the $M^{+}$ cation in the sequences $\mathrm{MX}$ which are discussed and compared with experiment.