Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations

TL;DR

This study uses first-principles calculations to analyze phonon frequencies in KNbO3 and KTaO3, showing good agreement with experiments and highlighting the effects of lattice volume on soft modes.

Contribution

It provides detailed first-principles phonon frequency calculations for KNbO3 and KTaO3, including effects of lattice volume on soft modes, which advances understanding of their lattice dynamics.

Findings

Calculated phonon frequencies agree with experimental data.

Lattice volume significantly affects the soft mode in KTaO3.

TO frequencies in KTaO3 are slightly higher than in KNbO3.

Abstract

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