Theory of bound polarons in oxide compounds - art. no. 184304

We present a multilateral theoretical study of bound polarons in oxide comp ounds MgO and alpha -Al2O3 (corundum). A continuum theory at arbitrary elec tron-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 absorpti on to nonrelaxed excited states. Unlike the case of free strong-coupling po larons, where the ratio kappa of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depe nds on the Frohlich coupling constant alpha and the screened Coulomb intera ction strength beta. Reasonable variation of these two parameters has demon strated that the magnitude of kappa remains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experim ental data for hole O- polarons bound to the cation vacancy in MgO. The the rmal dissociation energy for the ground self-consistent state and the energ y of the optically induced charge transfer process (hops of a hole between O2- ions) have been calculated using the quantum-chemical method INDO (inte rmediate neglect of the differental overlap). Results obtained within the t wo approaches for hole O- polarons bound by the cation vacancies (V-) in Mg O 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 ob tained on the basis of independent models and their agreement with experime nt.