The electrocaloric effect of lead-free Ba1−yCayTi1−xHfxO3 from direct and indirect measurements

We report on the dielectric and electrocaloric properties of Ba1−yCayTi1−xHfxO3 for compositions 0.12<x<0.18 and y=0.06, as well as x=0.15 and 0<y<0.15, synthesized by the conventional solid-state reaction method. The addition of Hf/Ca broadens the ferroelectric-paraelectric phase transition, while moving it toward room temperature. Two interferroelectric transitions are seen to converge, together with the ferroelectric-paraelectric phase transition, at ca. 335 K for 0.12<xc<0.135 and y=0.06. Consistently with the dielectric properties, the electrocaloric effect maximizes closer to room temperature with increasing Hf/Ca substitutions, which promote larger temperature spans. The electrocaloric responsivity gradually decreases from 0.2 to 0.1 K mm kV−1 with the addition of Hf/Ca. A homemade quasi-adiabatic calorimeter is employed to measure "directly" the electrocaloric data, which are also calculated from polarization-versus-electric-field cycles using "indirect" standard procedures. The comparison between measured and calculated values highlights the importance of having access to direct methods for a reliable determination of the electrocaloric effect.