ELECTRONIC-STRUCTURE OF KNBO3 AND KTAO3

The electronic structure of cubic KNbO3 and KTaO3 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 with 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 KTaO3 (2.25 Mbar) is in good agreement with experiment, while for KNbO3 (2.47 Mbar) it is nearly twice as large as the experimental value. Cohesive energies are found to be -42.2 eV for KNbO3 and -44.5 eV for KTaO3 (per unit cell). Corresponding experimental values do not seem to exist in standard literature.