Tight-binding total energy study of phase transitions in KNBO₃

Abstract

We have developed a tight-binding electronic structure framework with total-energy capabilities which includes an explicit treatment of charge transfer effects and non-point-ion interactions. In this framework, the total energy is composed of four parts: i) A sum over the band energies, which includes charge transfer effects in the self energies and an overlap matrix proportional to the Hamiltonian matrix, ii) an electron-electron double counting term, iii) a core-core interaction term, and iv) a term that produces Coulombic interactions, where, for first near neighbors, we include a penetration factor that serves to appropriately decrease screening effects. Electronic self consistency is established by updating the self-energy contributions to the Hamiltonian according to the results of a population analysis. Based upon these calculations, we present a potential-energy surface of potassium niobate with respect to symmetry lowering displacements. These results are consistent with a system that spontaneously distorts from a cubic configuration to a tetragonal one (a double well), which is in qualitative agreement with experiment.