Electronic structure ofKNbO3andKTaO3

The electronic structure of cubic ${\mathrm{KNbO}}_3$ and ${\mathrm{KTaO}}_3$ has been calculated using the self-consistent, scalar-relativistic linear-muffin-tin-orbital method. The calculated density of states (DOS) shows a strong similarity for both materials and is in good accordance to measured photoelectron spectra (PES). The projected DOS reveals a strong d-band character for the valence band, which is due to an evident hybridization of O 2p states with the unoccupied Nb (Ta) d states. This is also confirmed by PES data, if one makes use of the Cooper minimum for d bands. The calculation underestimates the band gaps by about 50%, a result that is known also from other band calculations for insulators within density-functional theory. Ground-state properties are obtained from total-energy calculations. Lattice constants agree within a few percent with experimental ones. The bulk modulus for ${\mathrm{KTaO}}_3$ (2.25 Mbar) is in good agreement with experiment, while for ${\mathrm{KNbO}}_3$ (2.47 Mbar) it is nearly twice as large as the experimental value. Cohesive energies are found to be-42.2 eV for ${\mathrm{KNbO}}_3$ and -44.5 eV for ${\mathrm{KTaO}}_3$ (per unit cell). Corresponding experimental values do not seem to exist in standard literature.