T − E phase diagrams and electrocaloric effect in PNN-PT solid solutions

Using the sixth-order free energy expansion of the Landay thermodynamic potential, T − E phase diagrams and the electrocaloric effect (ECE) in solid solutions (1-x)Pb(Ni1∕3Nb2∕3)O3-xPbTiO3 (PNN-PT) were studied under an electric field along the crystallographic directions [001], [101] and [111]. The composition of the samples, temperature range, as well as magnitude and direction of the electric field significantly affect the sequence of phase transitions, stability of various crystalline phases and electrocaloric properties. Direct measurements of the intensive ECE in the 0.8PNN-0.2PT compound were carried out by the method of adiabatic calorimetry at electric field strength E = 6 kV/cm. The large electrocaloric coefficient ΔT∕ΔE = 10 μK ⋅ cm/kV indicates that this compound and other PNN-PT solutions are promising for application as solid-state refrigerants. [Display omitted] •First thermodynamic analysis of T − E phase diagrams of PNN-PT solid solutions.•Strong effect of the electric field direction on the sequence of crystal phases.•Inverse electrocaloric effect along the [001] axis near the morphotropic boundary.•First direct measurements of the electrocaloric effect in PNN-PT.•Contribution of Joule heat to temperature change under the electric field.