First-principles-based calculation of the electrocaloric effect inBaTiO3: A comparison of direct and indirect methods

Abstract

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 ∆T and the isothermal entropy change ∆S, for different electric field strengths. We compare direct and indirect methods to obtain ∆T and ∆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 ∆T using the direct method within a molecular dynamics approach.

Influence of defects on ferroelectric and electrocaloric properties ofBaTiO3

“…In the following, technical aspects of the simulation will be discussed in detail, a comparison of the direct and indirect method will be reviewed from literature, 19 and additional convergency tests will be presented. In the present paper we calculate the caloric response of BTO directly, see Sec.…”

“…By this approach we can get a good qualitative description of the change of the ECE with different kind of defects as we take the most relevant degrees of freedom into account and induce similar errors without all our simulations. 6,18,19,25 Alternatively, the thermodynamic Maxwell relations (cf. Eqn.…”

“…The same model has been used successfully in order to model ferroelectric phase diagrams and the ECE in literature. 2,4,6,18,19 In particular, experimental and theoretical investigations reveal the important role of transition metal doping on fatigue behavior in ferroelectric perovskites. 12,[15][16][17] It has been found that an internal bias field builds up in field polarized (poled) Cr doped BTO, which could be related to the alignment of defect dipoles with the overall polarization.…”

Amplitudon and phason modes of electrocaloric energy interconversion

“…Instead of utilising the Maxwell relation (3), a number of approaches involving combinations of effective-Hamiltonian techniques with molecular dynamics or Monte Carlo simulations have been used to directly compute field-induced temperature and entropy changes in ferroelectrics. [47][48][49] (Such techniques can indeed handle polar and elastic spatial inhomogeneities when appropriately parameterised.) Although with no first-order phase transitions present both types of approaches should produce the same results, 49 the accuracy of the type adopted here may depend on precision of numerical integration of the Maxwell relations.…”

“…[47][48][49] (Such techniques can indeed handle polar and elastic spatial inhomogeneities when appropriately parameterised.) Although with no first-order phase transitions present both types of approaches should produce the same results, 49 the accuracy of the type adopted here may depend on precision of numerical integration of the Maxwell relations. 47 We have investigated convergence of the integral in Equation (4) for the poling schemes described above, with variations ⩽ 15-20% found for the resulting ΔT, as long as step of numerical sweeping of the poling field ΔE γ was not too small (typical step values ranged from 0.5 to 0.05 kV/cm).…”