PHONON PROPERTIES OF KNBO3 AND KTAO3 FROM 1ST-PRINCIPLES CALCULATIONS

The frequencies of transverse-optical GAMMA phonons in KNbO3 and KTaO3 are calculated in the frozen-phonon scheme making use of the full-potential linearized muffin-tin orbital method. The calculated frequencies in the cubic phase of KNbO3 and in the tetragonal ferroelectric phase are in good agreement with experimental data. For KTaO3, the effect of lattice volume was found to be substantial on the frequency of the soft mode, but rather small on the relative displacement patterns of atoms in all three modes of the T1u symmetry. The TO frequencies in KTaO3 are found to be of the order of, but somehow higher than, the corresponding frequencies in cubic KNbO3.