The electronic structures, Born effective charge tensors, and phonon properties of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5NbO3: A first-principles comparative study

The electronic structures, Born effective charges (BECs), and full phonon dispersions of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5NbO3 are investigated by the first principles method based on density functional theory. The hybridized states of Nb 4d and O 2p states are observed in the valence band, showing the formation of a strong Nb-O covalent bond which should be responsible for the displacement of Nb and O atoms. The abnormally large BECs of Nb and O indicate the possibility of phase instability induced by the off-center displacement of Nb and O atoms. The phonon dispersions reveal that the ferroelectric instability of K0.5Na0.5NbO3 is dominated by Nb and O displacements with significant Na characteristics. In addition to the ferroelectric instability, there is also rotational instability coming from the oxygen octahedra rotation around one axis. Moreover, the Gamma phonon properties of orthorhombic KNbO3, NaNbO3, and K0.5Na0.5NbO3 are also studied in detail.