First-principles study of structural, electronic, elastic, and optical properties of cubic KNbO3 and KTaO3 crystals

Cubic KNbO3 and KTaO3 crystals are studied using the first-principles VASP code. By employing local density approximation (LDA), GGA-PBE, and HSE06, the lattice parameters are optimized and compared with available experimental data, and the best agreement is achieved with HSE06. Electronic structures such as density of states, band structures, and charge-density distribution are discussed in detail for both crystals. The single-crystal elastic constants, polycrystalline elastic modulus, compressibility, Poisson's ratio, and anisotropy factors are obtained from the Voigt-Reuss-Hill approximation. The elastic anisotropy is modeled and visualized in the light of the elastic properties of both systems. The bandgaps of the two crystals are calculated with LDA, PBE, HSE06, and GW approximations (GW0), and the results of HSE06 and GW0 agree well with experimental data. Then the more reliable optical properties of KNbO3 and KTaO3 are acquired based on the bandgaps with HSE06, in which the previous scissors operator correction is avoided. Both crystals are brittle, and KTaO3 exhibits higher hardness and stiffness than KNbO3. Some novel results, such as Debye temperatures, sound velocities, and the extreme values of Young's modulus are obtained.