Phonon entropy in the electrocaloric effect of BaTiO3: A study based on dual-phase-transition mechanisms

Abstract

Electrocaloric effect refers to an induced adiabatic temperature change and/or isothermal entropy change that results from variations in electric polarization within a dielectric material under an applied electric field. The isothermal entropy change ΔS contains dipole entropy change ΔSdip due to dipole alignment and phonon entropy change ΔSph due to intrinsic structure response. As the temperature increases, the typical ferroelectric material BaTiO3 undergoes a series of phase transitions from rhombohedral, orthorhombic, and tetragonal ferroelectric phase to cubic paraelectric phase. Moreover, the phase transition mode is the coexistence of displacive and order–disorder type. Prior studies have predominantly focused on the electrocaloric effect in the displacive model. Here, we thoroughly investigate the phonon entropy change ΔSph and phonon temperature change ΔTph based on both phase transition models and three ferroelectric phases and evaluate their contribution to the total electrocaloric effect. Our findings reveal that ΔTph contributes up to 45% of the total electrocaloric effect in the rhombohedral phase, representing the greatest average contribution among the phases. Furthermore, we employ the phonon density of states to elucidate the mechanisms of ΔSph under external electric fields. We also provide insights into the dynamic charge transfer of strongly coupled atoms under electric field through Born effective charge.