Stabilization of the first-order phase transition character and enhancement of the electrocaloric effect by Na_0.5Bi_0.5TiO₃ substitution in BaTiO₃ ceramics

The electrocaloric properties of BaTiO₃-based Pb-free ferroelectric materials are widely investigated. One approach to achieving a large electrocaloric response is making use of the substantial polarization change associated with the first-order phase transition at the Curie temperature. To make use of this approach, we have investigated the electrocaloric response of (1 − x)BaTiO₃–xNa_0.5Bi_0.5TiO₃ (BT–NBT) ceramics for x = 0.05, 0.10, 0.20 and 0.30. For this BT-rich part of the solid solution, it is established that increasing the NBT content increases the tetragonality of the BaTiO₃. We show that this increase in tetragonality with NBT substitution helps to maintain the first-order nature of the phase transition in BaTiO₃ and correspondingly a large electrocaloric response, despite the simultaneous enhancement of relaxor ferroelectric character with the NBT substitution. A significantly larger effective electrocaloric temperature change (ΔT_eff) of 1.65 K was obtained for the x = 0.20 sample under 40 kV cm⁻¹, using the direct measurement of the electrocaloric effect, which is in reasonable agreement with the indirect measurements.