Effects of Long- and Short-Range Ferroelectric Order on the Electrocaloric Effect in Relaxor Ferroelectric Ceramics

We study the electrocaloric effect (ECE) in a typical relaxor ferroelectric ceramic of Pb0.91La0.06Zr0.8Ti0.2O3 by analyzing the respective contributions of long-range-ordered ferroelectric macrodomains and short-range-ordered polar nanoregions (PNRs) by both direct and indirect characterization. The ECE exhibits two peaks in its dependence on temperature, the former responds to the transition from long-range ferroelectric order to short-range-ordered PNRs and the latter likely to the transition between two kinds of PNRs with different phase structures or motion states. The contributions of different mechanisms are clarified by resolving the overlapping peaks in direct heat flow experimental results. The first peak has a ΔTmax=0.7K at 80 °C (approximately Td) and the second a ΔTmax=0.98K at 140 °C (approximately Tm) under 40 kV/cm. We conclude that the thermodynamic method based on the Maxwell relation is not applicable if PNRs are involved as their signatures are only picked up by direct measurements, but is valid for the case of ferroelectric domain rotation or phase transition where it provides a reasonably accurate ECE result. Besides the impact on aspects related to metrology, our work suggests that the region between the peaks in ΔT-T curves may be used to produce a large electrocaloric ΔT over a broad temperature range, which is highly desirable in cooling applications.