Understanding the enhanced electrocaloric effect in BaTiO3-based ferroelectrics at critical state

Abstract

Ferroelectrics exhibiting effective couplings of polarization with temperature and electric field are promising candidates for electrocaloric (EC) refrigeration. BaTiO3 (BT) as one of the most widely investigated lead-free ferroelectrics, its enhanced EC performance is usually achieved near the so-called “critical state”. Here, for the first time, different critical compositions and the correlation between critical state and EC effect are systematically investigated. For critical components Ba(M0.11Ti0.89)O3 (BT-0.11M, M=Zr, Hf, Sn), TC reduces at divergent rates, meaning different degrees of frustration on ferroelectric correlations, thereby discrepant EC performance is shown. For BT-0.11Sn, the superior room-temperature EC performance of ∼0.9 K is displayed, which is comparable to BT-0.11Hf with much higher TC. However, maximum ΔT of BT-0.11Hf is lower than that of BT-0.11Zr even with similar TC. The same critical state makes them the similar phase structure, while different domain contribution is found via Rayleigh analysis of reversible and irreversible contributions. For samples more relaxor-liked, more local heterogeneity and highly dynamic polar nano-regions can expand the operating temperature, and compensate the frustrated long-range ferroelectric correlations effectively, leading to enhanced ΔT. This work establishes the inner correlation between critical state and EC effect, and also provides a general principle for designing practical high-performance EC materials via intentionally introducing moderate relaxation.