8 - First-principles study of the ferroelectric phase of AgNbO3

Abstract

AgNbO3 exhibits an antiferroelectric phase transition to the orthorhombic Pbcm phase at ~626K. Recently, Fu et al. reported on the ferroelectric behavior of this compound at room temperature. However, its ferroelectric structure is still not well understood. Levin et al. reported the ferroelectric phase of AgNbO3 also has Pbcm symmetry, attributing its ferroelectricity to local symmetry breaking and/or a defective structure. More recently, Yashima et al. proposed space group Pmc21 for the ferroelectric phase based on converged beam electron diffraction and neutron and X-ray diffraction results. Here we report first-principles calculations of the ferroelectric phase of AgNbO3 using the projector augmented wave method based on density functional theory. The calculated phonon dispersion curve of the Pbcm phase shows that no soft-mode is evident over all wave vectors, indicating that the Pbcm phase is a dynamically stable structure. The ferroelectric behavior of AgNbO3 thus cannot be explained based on this calculation. Extensive calculations are under way to uncover the correct ferroelectric transition mechanism of this compound.