First-principles-based calculation of the electrocaloric effect in BaTiO3: A comparison of direct and indirect methods

We use molecular dynamics simulations for a first-principles-based effective Hamiltonian to calculate two important quantities characterizing the electrocaloric effect in BaTiO3, 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 an 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 consider the full temperature and field dependence of the calculated specific heat for the indirect calculation of Delta T, and 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.