Theory of bound polarons in oxide compounds

Abstract

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and α-Al 2 O 3 (corundum). A continuum theory at arbitrary electron-phonon coupling is used for calculation of the energies of thermal dissociation, photoionization (optically induced release of an electron (hole) from the ground self-consistent state), as well as optical absorption to the 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 O 2− ions) have been calculated using the quantum-chemical method INDO. Results obtained within the two approaches for hole O − polarons bound by the cation vacancies (V − ) in MgO and by the Mg 2+ impurity (V M g ) 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.

Oxygen interstitial structures in close-packed metal oxides

“…Furthermore, many theoretical studies of defects in Al 2 O 3 have been reported based on classical interatomic potential [6][7][8][9][10], semi-empirical [11][12][13], Hartree-Fock [14], density functional [15,16], and most recently hybrid density functional methods [17,18]. Many of these studies have focused on the F-centre associated with oxygen vacancy formation, while the nature of oxygen interstitial structure and spin configuration has received much less attention.…”

Electron Correlations and Spatial Configuration of the Bipolaron

“…Recently the interest in the polaron and bipolaron subject field has surfaced again due to an expansion of this field of investigations to anisotropic crystals, low dimensional structures and systems with quantum wells [7,[10][11][12][13]. The problem of the choice of a bipolaron spatial configuration and consideration of electron correlations is actual for such systems, too.…”

Two-Dimensional Polaronic Behavior in the Binary Oxidesm−HfO2andm−ZrO2

“…Quasi-two-dimensional (2D) systems, including surfaces, heterointerfaces and layered materials, have become a rich playground for discovering exotic electronic effects such as high T c -superconductivity and novel magnetic phases [1][2][3][4][5]. The formation of polarons and the modification of their properties by reduced dimensionality have been implicated in many of these phenomena [1,[6][7][8]. Although a number of systems, including layered transition-metal oxides [5], surfaces and thin insulating films [9,10] are suspected to exhibit 2D polaronic behavior, directly probing their properties remains extremely challenging.…”