Structural, elastic, electronic and optical properties of Cubic NaNbO3 crystals under pressure

The structural, elastic, electronic and optical properties of cubic NaNbO3 (c-NNO) crystals in the pressure range 0-20 GPa are studied using first-principles VASP code. The influences of pressure on lattice constants, unit cell volume, elastic constants, elastic modulus, elastic anisotropy, sound velocities, elastic wave velocities, Debye temperature and Gruneisen parameters are discussed. Electronic structure calculations show that c-NNO is a wide indirect bandgap semiconductor. The band structures are similar for different pressures, except that conduction band shifts toward higher energy with increasing pressure. The distributions of density of states reveal typically covalent Nb-O bonding with strong hybridizations and significantly ionic Na-O ones. The dielectric function and linear optical properties are calculated with HSE06 scheme, and the pressure effects of the optical properties are also investigated. The optical spectra of c-NNO exhibit similar shapes for distinct pressures and blueshifts with the increase of pressure. The phonon dynamical properties are also investigated from the density functional perturbation theory (DFPT), and the calculated phonon spectra exhibit some imaginary frequencies, which reflect the negative phonon modes and hence the dynamical instability of the system.