Emergence of central mode in the paraelectric phase of ferroelectric perovskites

TL;DR

This study combines THz dielectric spectroscopy and molecular dynamics simulations to reveal a crossover in dielectric behavior of paraelectric ferroelectric perovskites, highlighting the emergence of a central mode linked to thermally activated processes.

Contribution

It introduces a unified phenomenological model explaining the dielectric response across the entire paraelectric phase, connecting soft mode and central mode dynamics.

Findings

Identification of a crossover between soft mode and coupled soft/central mode regimes

Development of a single model fitting the dielectric spectra across temperatures

Linking the central peak to thermally activated processes

Abstract

THz-range dielectric spectroscopy and first-principle-based effective-Hamiltonian molecular dynamics simulations were employed to elucidate the dielectric response in the paraelectric phase of (Ba,Sr)TiO3 solid solutions. Analysis of the resulting dielectric spectra suggests the existence of a crossover between two different regimes: a higher-temperature regime governed by the soft mode only versus a lower-temperature regime exhibiting a coupled soft mode/central mode dynamics. Interestingly, a single phenomenological coupling model can be used to adjust the THz dielectric response in the entire range of the paraelectric phase (i.e., even at high temperature). We conclude that the central peak is associated with thermally activated processes, and that it cannot be discerned anymore in the dielectric spectra when the rate of these thermally activated processes exceeds certain characteristic frequency of the system.