Phonon anharmonic investigation on the different structural phase transition processes of cubic KNbO3 and KTaO3

Abstract

KTaO3 and KNbO3, two typical perovskite ferroelectric crystal materials with identical cubic structure at high temperature, undergoes totally different phase transition path. The underlying mechanism is explored by comparing their phonon anharmonic behavior based on first-principle calculation. The results of Grüneisen parameters indicate larger phonon anharmonicity of KNbO3 than that of KTaO3, which leads to structural unstable of KNbO3. Different with previous studies of soft mode theory, apparent phonon anharmonicity of both crystals are stemmed not from the center, but the corner and boundary of the first Brillouin zone. In order to essentially understand the difference in the anharmonic behavior of phonons, the chemical bond strength of these two perovskite materials was analyzed by ELF. The results show that due to the stronger covalent properties of Ta-O bond, it will cause weak phonon anharmonicity, and ultimately lead to high phonon relaxation time and low Grüneisen parameters of KTaO3. The weakly ionic Nb-O bond give rise to the obvious phonon anharmonic behavior and lattice instability of KNbO3, resulting in a series of temperature-dependent phase transitions.

Keywords

• Phase transition
• Phonon relaxation
• Grüneisen parameter
• Phonon anharmonicity
• ELF
• Soft mode