Crystallographic phase stabilities and electronic structures in AgNbO3 by first-principles calculation

Abstract

Silver niobate (AgNbO3) assumes several phases that crystallise in the perovskite structure similar to sodium niobate (NaNbO3). In order to investigate the phase stability of AgNbO3, the formation enthalpies of both physically the realised (real) cubic , tetragonal (P4/mbm) and orthorhombic (Cmcm, Pbcm) phases as well as the virtual phases, the orthorhombic (Pc21 b) and rhombohedral (R3cR) phases, which are observed in NaNbO3, have been obtained using a plane-wave pseudopotential method. Although the orthorhombic (Pbcm) phase has the lowest formation enthalpy among the real phases, the virtual rhombohedral (R3cR) phase has the lowest symmetry as well as the lowest formation enthalpy among all phases. We thus speculate that the rhombohedral (R3cR) phase may be stable at extremely low temperatures provided that it is accessible kinetically. In addition, the electronic structures of the various phases of AgNbO3 are calculated within the generalised gradient approximation with the Perdew–Burke–Ernzerhof correction (GGA-PBE). The valence band top was found to consist of predominately localised Ag 4d and O 2p orbitals constituting the O 2p–Nb 4d bonding orbital, while the bottom of the conduction band was found to mainly consist of the anti-bonding orbital of Nb 4d–O 2p under the Ag 5s orbital.