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

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In order to clarify the thermodynamic relations of various phases in NaNbO3, the enthalpies of formation for the cubic (\textit{Pm$\bar{\textit{3}}$m}), 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.
2004-09-15