First principles-based calculation of the electrocaloric effect in BaTiO$_3$: comparison between direct and indirect methods

TL;DR

This study employs molecular dynamics simulations with a first principles-based effective Hamiltonian to compare direct and indirect methods for calculating the electrocaloric effect in BaTiO$_3$, confirming their equivalence under certain conditions and highlighting the large electrocaloric response near phase transitions.

Contribution

It demonstrates the consistency of direct and indirect methods for electrocaloric calculations in BaTiO$_3$ using a first principles-based approach and discusses key methodological considerations.

Findings

Both methods yield identical results when no first order phase transition occurs.

Large electrocaloric response observed beyond the critical field for phase transition.

Maintaining thermal equilibrium is crucial during field ramping in direct method calculations.

Abstract

We use molecular dynamics simulations for a first principles-based effective Hamiltonian to calculate two important quantities characterizing the electrocaloric effect in BaTiO$_3$, the adiabatic temperature change $\Delta T$ and the isothermal entropy change $\Delta S$, for different electric field strengths. We compare direct and indirect methods to obtain $\Delta T$ and $\Delta S$, and we confirm that both methods indeed lead to identical result provided that the system does not actually undergo a first order phase transition. We also show that a large electrocaloric response is obtained for electric fields beyond the critical field strength for the first order phase transition. Furthermore, our work fills several gaps regarding the application of the first principles-based effective Hamiltonian approach, which represents a very attractive and powerful method for the quantitative prediction of electrocaloric properties. In particular, we discuss the importance of maintaining thermal equilibrium during the field ramping when calculating $\Delta T$ using the direct method within a molecular dynamics approach.