Electrocaloric Effect of Perovskite High Entropy Oxide Films

This paper describes two perovskite high entropy oxide (PHEO) compositions: Pb(Hf_0.2Zr_0.2Ti_0.2Nb_0.2Mn_0.2)O₃ (Mn PHEO) and Pb(Hf_0.2Zr_0.2Ti_0.2Nb_0.2Al_0.2)O₃ (Al PHEO). Powders are prepared by conventional solid state sintering by first pre-reacting the B-site oxides, then adding PbO. Phase pure Mn PHEO powder is obtained following calcination of the mixed powders at 750 °C for 240 min; however, secondary phases persisted in Al PHEO for heat treatments from 750 °C to 1200 °C. The Mn PHEO undergoes an entropy-driven phase transformation. Thin films of these compounds are synthesized by pulsed laser deposition (PLD) on a lead zirconate titanate seed layer on Pt-coated SiO₂/Si. The dielectric response of the Mn PHEO films show some contribution from space charge polarizability; in contrast, the Al PHEO films show a slim ferroelectric hysteresis loop and relaxor-like characteristics. The Al PHEO has a dielectric permittivity of ≈2000 with a loss tangent <0.05 from 100 Hz to 100 kHz; it has a dielectric maximum at 105 ± 0.5 °C and a Burns’ temperature of 234 ± 0.5 °C. Indirect measurements based on the Maxwell-relations yielded a maximum electrocaloric temperature change of 8.4 K at 180 °C under the applied electric field of 1186 kV cm⁻¹.