Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (corundum). A continuum theory at arbitrary electron-phonon coupling is used for the calculation of the energies of thermal dissociation and photoionization [optically induced release of an electron (hole) from the ground self-consistent state], as well as optical absorption to nonrelaxed excited states. Unlike the case of free strong-coupling polarons, where the ratio \ensuremath{\kappa} of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Fr\"ohlich coupling constant \ensuremath{\alpha} and the screened Coulomb interaction strength \ensuremath{\beta}. Reasonable variation of these two parameters has demonstrated that the magnitude of \ensuremath{\kappa} remains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole ${\mathrm{O}}^{\ensuremath{-}}$ polarons bound to the cation vacancy in MgO. The thermal dissociation energy for the ground self-consistent state and the energy of the optically induced charge transfer process (hops of a hole between ${\mathrm{O}}^{2\ensuremath{-}}$ ions) have been calculated using the quantum-chemical method INDO (intermediate neglect of the differental overlap). Results obtained within the two approaches for hole ${\mathrm{O}}^{\ensuremath{-}}$ polarons bound by the cation vacancies $({\mathrm{V}}^{\ensuremath{-}})$ in MgO and by the ${\mathrm{Mg}}^{2+}$ impurity $({\mathrm{V}}_{\mathrm{Mg}})$ in corundum are compared to experimental data and to each other. We discuss a surprising closeness of the results obtained on the basis of independent models and their agreement with experiment.