Enhanced electrocaloric effect near polymorphic phase boundary in lead-free potassium sodium niobate ceramics

The electrocaloric (EC) effect in lead-free (1-x)(K_0.48Na_0.52)(Nb_0.95Sb_0.05)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃ ceramics was investigated using an indirect thermodynamic method. Large EC temperature changes were obtained in the vicinity of a polymorphic phase boundary at 40 kV/cm, e.g., 0.32 K at 359 K for x = 0.03, 0.51 K at 350 K for x = 0.04, and 0.48 K at 300 K for x = 0.05, respectively. These values are larger than the previous results at inter-ferroelectric phase transition and, more interestingly, are found to be comparable to those usually explored at the Curie temperature. The operational temperature window is broad near the polymorphic phase boundary due to the diffuseness of the phase transition. The enhanced electrocaloric effect is attributed to the formation of nanodomains near the polymorphic phase boundary, which reduces domain wall energy and facilitates the polarization rotation. The construction of a polymorphic phase boundary and the arrangement of coexisting phases at the nanoscale may open a promising route to explore EC materials.