Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and alpha -Al2O3 (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 kappa of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Frohlich coupling constant alpha and the screened Coulomb interaction strength beta. Reasonable variation of these two parameters has demonstrated that the magnitude of kappa remains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole O- 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 O2- ions) have been calculated using the quantum-chemical method INDO (intermediate neglect of the differental overlap). Results obtained within the two approaches for hole O- polarons bound by the cation vacancies (V-) in MgO and by the Mg2+ impurity (V-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.