Electrocaloric effect in pure and NaNbO3-modified Na0.5Bi0.5TiO3-BaTiO3 solid solutions near the morphotropic phase boundary

Morphotropic phase boundary of ferroelectric solid solutions attracts interest with respect to not only improved piezoelectric properties but also the electrocaloric effect, which is attractive to build new-generation cooling devices. In the present study, the electrocaloric effect in (1 − x − y)Na_0.5Bi_0.5TiO₃-xBaTiO₃-yNaNbO₃ near its morphotropic phase boundary is studied by direct measurements of temperature change ΔT. ΔT maximum is observed in the region of depolarization temperature, where the major contribution comes from an entropy jump at the electric field-induced phase transition. Differences between ΔT values measured when an electric field pulse is applied and removed are explained by slow domain rearrangement in the ferroelectric phase and metastability of phases close to the depolarization temperature. Among studied compositions, the highest value of ΔT is obtained for the compositions 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ and 0.93Na_0.5Bi_0.5TiO₃-0.06BaTiO₃-0.01NaNbO₃ in the region of depolarization temperature. Examples of the comparison of the directly measured values of ΔT and the values calculated using the Maxwell relation revealing inconsistency between the two methods in the ferroelectric and the nonferroelectric phase are presented.