Enthalpy of Formation of Various Phases and Formation Energy of Point Defects in Perovskite-Type NaNbO₃ by First-Principles Calculation

In order to clarify the thermodynamic relations of various phases in NaNbO₃, the enthalpies of formation for the cubic (Pm3m), tetragonal (P4/mbm), orthorhombic (Ccmm, Pnmm, Pbma) and rhombohedral (R3c) phases in NaNbO₃ were calculated using a plane-wave pseudopotential method. The NaNbO₃ phase with a lower symmetry has a smaller enthalpy of formation. The enthalpy of formation for the rhombohedral (R3c) phase is the smallest. Moreover, in order to quantitatively evaluate the formation energies of point defects in NaNbO₃, first-principles pseudopotential calculations using plane-wave basis functions were performed. Relaxation of first- and second-neighbor atoms around a vacancy was considered in a 40-atom supercell. The formation energies of point defects were calculated as a function of the atomic chemical potentials of constituent elements. The formation energy of Na vacancy was the lowest under the oxidation limit of NaNbO₃ and that of O vacancy was the lowest under the reduction atmosphere. The formation energy of Nb vacancy was the highest under both oxygen-rich and -poor conditions. This result agrees with the empirical rule that the B site defect in the perovskite-type oxide does not exist.