Effects of Hydrostatic Pressure on the I–II Phase Transition in an Improper Ferroelectric NH₄LiSO₄

Abstract

This paper is concerned with the hydrostatic pressure dependence of dielectric properties and the results of the thermal expansion measurements around the I–II phase transition (186.5°C at 1 atm) of NH₄LiSO₄. The observed pressure coefficient of the transition temperature is dT_1c⁄dp=9.8 deg/kbar from which the volume electrostrictive coefficient is obtained to be Σ_j=1³Q_1j^*=−4.1×10⁻⁹ cgs esu. The calculated value of spontaneous volume change at T_1c is in good agreement with the experimental value of V_s=−6×10⁻⁴. The pressure dependence of the Curie constant and the temperature dependence of V_s are discussed by the help of a phenomenological theory including the latent transition parameters associated with prototype→I→II phase transitions.