Direct and indirect studies of the electrocaloric effect in single crystalline ferrielectric (NH4)(2)SO4

The influence of an electric field on the phase transition parameters and the electrocaloric response in ferrielectric (NH₄)₂SO₄ was studied using a universal multifunctional adiabatic calorimeter. The very low sensitivity of the phase transition temperature to the electric field, dT₀/dE ≈ 5·10^-3 K/(kV/cm), was shown to be the reason for the low maximum value of the intensive electrocaloric effect, ΔT_AD, at E = 11 kV/cm: direct and indirect measurements gave the values of 0.01 K and 0.05 K, respectively. However, the extensive electrocaloric response revealed by an analysis of the entropy–temperature–electric field phase diagram was relatively large, with a value of ΔS_ECE ≈ − 19 J/kg K, due to the significant contribution of a jump in the excess entropy to the total value, δS₀/ΔS₀ ~ 0.37. An additional irreversible increase in temperature under the electric field was associated with the Joule effect and indicated a significant electrical conductivity of ammonium sulphate.