Electrocaloric effect enhancement in compositionally graded ferroelectric thin films driven by a needle-to-vortex domain structure transition

Obtaining large electrocaloric (EC) effects in ferroelectric materials is a prerequisite for incorporating them into advanced solid-state cooling devices. Based on phase-field simulations, we propose an effective approach for improving the EC effect near room temperature in compositionally graded ferroelectric (CGFE) thin films (e.g., Pb_1 − xSr_xTiO₃) by controlling the compositional gradient along the direction of the thickness of the films. The simulation results reveal the formation of a stable needle-like domain structure in CGFE films with large composition gradients, while a periodic striped domain structure is found in small-composition-gradient CGFE films. The temperature dependence of polarization can be tailored by controlling the composition gradient in the films. With rising temperatures, a notable transition from a needle-like to a vortex domain structure is observed in CGFE films, which is distinguishable from that observed in homogeneous ferroelectric films. Our work demonstrates that the EC effect (the adiabatic temperature change, ΔT) can be greatly improved by the needle-to-vortex domain transition that occurs when the composition gradient is increased. When the composition gradient increases, a larger EC effect is achieved at lower temperatures. Large-composition-gradient films exhibit multiple peaks of ΔT. The local concentration of the total energy near the root of the needle domains is also found to drive the needle-to-vortex domain transition, giving rise to an enhancement of the EC effect in CGFE films. Our study provides a potential pathway for designing ferroelectric thin films with enhanced EC properties for energy-efficient solid-state refrigeration. (paper)