Antiferroelectric orthorhombic P -to- R phase transition induced exotic electrocaloric effect in NaNbO₃-based lead-free ceramics

Phase transition is expected in designing high-performance electrocaloric effect (ECE). However, hidden antiferroelectricity in NaNbO3-based ceramics complicates ECE evolution as compared to conventional ferroic materials. Therefore, a NaNbO3-xBi0.5Li0.5TiO3 (x = 0.08, 0.10, and 0.12) system is introduced to explore ECE performance in P (Pbma), Q (P21ma), and R (Pnma) phases since a Bi0.5Li0.5TiO3 dopant plays a Janus role in stabilizing ferroelectric Q (x = 0.08 and 0.10) and inducing the antiferroelectric (AFE) relaxor R phase (x = 0.12). Rietveld refinement reveals a phase structure change from the P/Q to R state. Bi0.5Li0.5TiO3 addition makes a downshift of P-to-R transition temperature accompanied by an increase in relaxation property, as certified by dielectric analysis/piezoelectric force images, in situ XRD/Raman spectra, and ferroelectric tests. Additionally, a square P-E loop evolves into less-sharp and finally changes into a slant one with increasing Bi0.5Li0.5TiO3 content. ECE is directly measured, and its peak value increases from 0.15 K for x = 0.08 and 0.21 K for x = 0.10 and finally to 0.11 K for x = 0.12 composition. Interestingly, dual ECE peaks appear in x = 0.10, and it is probably contributed by R relaxor polar nanodomains. This work not only clarifies actual ECE behavior in AFE orthorhombic (P/R) NaNbO3-based ceramics but also fulfills understandings of AFE P-to-R transitions in modulating ECE.