Positive and negative electrocaloric effect in BaTiO$_3$ in the presence of defect dipoles

TL;DR

This study investigates how defect dipoles influence the electrocaloric effect in BaTiO$_3$, revealing conditions for positive and negative ECE and proposing a cycle to enhance cooling using defect dipoles.

Contribution

It provides a detailed analysis of defect dipoles' impact on ECE in BaTiO$_3$ using Monte Carlo simulations and introduces a novel cycle leveraging negative ECE for improved cooling.

Findings

Anti-parallel defect dipoles can induce positive or negative ECE depending on their density.

A transition from negative to positive ECE occurs with increasing external field at certain defect densities.

The proposed electrocaloric cycle utilizes negative ECE and defect dipoles to enhance cooling performance.

Abstract

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO$_3$ is studied by means of lattice-based Monte-Carlo simulations. A Ginzburg-Landau type effective Hamiltonian is used. Oxygen vacancy-acceptor associates are described by fixed defect dipoles with orientation parallel or anti-parallel to the external field. By a combination of canonical and microcanoncial simulations the ECE is directly evaluated. Our results show that in the case of anti-parallel defect dipoles the ECE can be positive or negative depending on the density of defect dipoles. Moreover, a transition from a negative to positive ECE can be observed from a certain density of anti-parallel dipoles on when the external field increases. These transitions are due to the delicate interplay of internal and external fields, and are explained by the domain structure evolution and related field-induced entropy changes. The results are compared to those obtained by MD simulations employing an {\it{ab initio}} based effective Hamiltonian, and a good qualitative agreement is found. In addition, a novel electrocaloric cycle, which makes use of the negative ECE and defect dipoles, is proposed to enhance the cooling effect.