Tunable and anomalous electrocaloric behaviors in Bi0.5Na0.5TiO3-based relaxor enabled by dynamics of polar nanoregions

Abstract

Eco-friendly electrocaloric (EC) refrigeration technology, by virtue of its high efficiency and a cutting-edge decarbonization strategy, has become a promising alternative for vapor compression refrigeration. The EC effect in Bi_0.5Na_0.5TiO₃-based ferroelectric is prominent since their high ferroelectricity and flexible phase structure regulation. However, the underlying mechanisms responsible for high and anomalous EC performances in Bi_0.5Na_0.5TiO₃-based ceramics are not fully addressed. In this work, a high ΔT with 0.66–0.94 K and a large temperature span (ΔT_span) of 50–60 K are achieved in Bi_0.5Na_0.5TiO₃-Ba(Ti, Hf)O₃-NaNbO₃ system. The EC effect exhibits a tunable behavior, i.e., the temperature corresponds to the maximal ΔT value (ΔT_max) gradually decreases via downshifting critical freezing temperature (T_f). This phenomenon is accompanied by the collapse of domains into polar nanoregions (PNRs) and a reduction in correlation length, as supported by inverse fast Fourier transformation analysis and piezoelectric force microscopy images. Furthermore, the asymmetrical EC profiles with a large gap between exothermic and endothermic EC peaks near T_f are largely ascribed to the time effect for PNRs evolution, as demonstrated by time-resolved pulsed dielectric spectra and customized polarization test. Interestingly, high ΔT is maintained as T ≥ T_f but sharply declines as T < T_f, which originates from a crossover from activated to frozen ferroelectric state. These findings reveal that PNRs dynamics strongly impact on EC performances, with high ΔT and broad ΔT_span primarily arising from highly polar and mobile PNRs. This work not only gives a deep insight into EC behaviors in Bi_0.5Na_0.5TiO₃-based ceramics but also proposes a refreshing strategy for optimizing EC performance with regulating PNRs dynamics.