Phonon properties ofKNbO3andKTaO3from first-principles calculations

The frequencies of transverse-optical Γ phonons in ${\mathrm{KNbO}}_3$ and ${\mathrm{KTaO}}_3$ 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 ${\mathrm{KNbO}}_3$ and in the tetragonal ferroelectric phase are in good agreement with experimental data. For ${\mathrm{KTaO}}_3$, 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 ${\mathit{T}}_{1\mathit{u}}$ symmetry. The TO frequencies in ${\mathrm{KTaO}}_3$ are found to be of the order of, but somehow higher than, the corresponding frequencies in cubic ${\mathrm{KNbO}}_3$.