Charge instabilities in the ionic model of metal oxides: Importance of polarization energy

Abstract

An ionic representation for metal oxides is used to investigate charge instability. The justification for our simple approach is the good agreement with a large body of experimental data. We demonstrate that useful insights and predictions applicable to a wide range of simple and complex oxides can be obtained with well-defined input data using no adjustable parameters. We find the conditions for which they are unstable with respect to charge disproportionation, ${M}^{z+}{+M}^{z+}\ensuremath{\rightarrow}{M}^{(z\ensuremath{-}1)+}{+M}^{(z+1)+}.$ Stability or instability for the charge disproportionation reaction is parametrized by ${U}_{d},$ given by the difference between the stabilizing energy $\ensuremath{\Delta}{I}_{z}$ ${(I}_{z}$ is the ionization potential) and the destabilizing energy which is the sum of the polarizing energies $\ensuremath{\Sigma}{H}_{p}.$ We have calculated the disproportionation energy. We further discuss consequences of the disproportionation instability including nonexistence of the oxidation state, nonstoichiometry, vacancy formation, metal-insulator behavior, and pairing interactions in the charge reservoir layers of the high-${T}_{c}$ cuprate superconductors.