Dynamic mechanisms of the structural phase transitions in KNbO 3: Molecular dynamics simulations

TL;DR

This study uses molecular dynamics simulations to investigate the mechanisms driving structural phase transitions in KNbO3, revealing the nature of dynamics and local polarization correlations across phases.

Contribution

We developed a shell model fitted to first-principles data and analyzed the phase diagram and microscopic dynamics to clarify the displacive versus order-disorder mechanisms.

Findings

Identified the dominant mechanisms at each phase transition.

Revealed chain-like precursor polarization clusters in the paraelectric phase.

Mapped the temperature-dependent phase diagram.

Abstract

The question on the dominant driving mechanism (displacive or order-disorder) at each structural phase transition of KNbO3 is investigated by means of molecular dynamics simulations. To this purpose, we first develop a shell model by determining its potential parameters in order to reproduce the ferroelectric instabilities obtained by first-principles total energy calculations. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations. The analysis of the dynamical structure factor and the microscopic dynamics of the particles in the different phases allows us to reveal the nature of the dynamics associated with each structural transition. Correlations between local polarizations forming chain-like precursor clusters in the paraelectric phase are examined.