Enhanced electrocaloric effect in lead-free relaxor ferroelectrics via point defect engineering

The electrocaloric effect enables solid-state refrigeration technology with high energy efficiency and zero global warming potential. Normal displacive ferroelectric materials exhibit the highest electrocaloric effect just above their Curie point. The freezing temperature (Tf) of a relaxor ferroelectric material functions as a Curie point in displacive ferroelectrics, suggesting that the highest electrocaloric effect for a relaxor ferroelectric material is near its Tf. However, relaxor ferroelectrics often exhibit significant hysteresis around Tf, making them unsuitable for electrocaloric applications. In this work, we show that the highest electrocaloric effect in lead-free Bi0.5Na0.5TiO3-SrTiO3-based relaxor ferroelectric ceramics occurs at temperatures a few degrees above the temperature with their maximum permittivity (Tm) and is far above their Tf. Moreover, the A-site vacancy design can be used to effectively increase the electrocaloric effect and broaden the working temperature range, which is due to the increase in field-induced polarization and strong relaxation achieved via point defect engineering. This Letter reveals the appropriate temperature window for the high electrocaloric effect in relaxor ferroelectrics and shows that the A-site vacancy design can further enhance electrocaloric properties and extend the working temperature range for electrocaloric applications.