Phase transitions in KNbO3: symmetry analysis and first principles calculations

TL;DR

This study combines group theoretical analysis and first-principles calculations to investigate the phase transitions in KNbO3, revealing stability only for the low-temperature rhombohedral phase and providing insights into the symmetry and stability of higher-temperature phases.

Contribution

It offers a comprehensive symmetry analysis and ab initio calculations of KNbO3 phases, clarifying the stability and transition mechanisms between different crystal structures.

Findings

Rhombohedral phase is stable at low temperature (263 K).

Higher temperature phases show imaginary phonon frequencies indicating nonstability.

Only the cubic-tetragonal transition is symmetry-allowed.

Abstract

Group theoretical aspects of the three temperature-dependent and temperature-reversible experimentally observed phase transitions in the KNbO3 crystal (cubic-tetragonal, tetragonal-orthorhombic, orthorhombic-rhombohedral) in the framework of the group-subgroup relationship tree have been discussed. The ab initio DFT-HSE06 LCAO calculations of the electron and phonon properties, with optimisation of lattice parameters and atomic coordinates for all experimentally observed KNbO3 phases, are used for better understanding of the details of these phase transitions. Good agreement with the experimental data was found for the structural properties. Ab initio calculations of the phonon dispersion curves confirmed the existence of a stable phase only for the rhombohedral structure found experimentally for the lowest temperature of 263 K. For the remaining three higher temperature phases, imaginary frequencies appear, implying a nonstability of these phases. The only the cubic-tetragonal phase transition has been found to be symmetry allowed. The tetragonal-orthorhombic and orthorhombic-rhombohedral phases are not related to the group-subgroup relationship. An explanation is proposed based also on the results of ab initio calculations of the structure of the monoclinic phase, which we have chosen as a virtual one for the tetragonal-orthorhombic transition in the bulk.