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 NH 4 LiSO 4 . The observed pressure coefficient of the transition temperature is d T 1 c /d p =9.8 deg/kbar from which the volume electrostrictive coefficient is obtained to be Σ 3 j =1 Q * 1 j =-4.1×10 -9 cgs esu. The calculated value of spontaneous volume change at T 1 c 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.