Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and α − 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 κ of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Fröhlich coupling constant α and the screened Coulomb interaction strength β. Reasonable variation of these two parameters has demonstrated that the magnitude of κ 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.