Effects of pressure on the dielectric properties of and the vanishing of the ferroelectricity in SbSI

Abstract

The effects of pressure and temperature on the dielectric properties and ferroelectric (FE) transition in SbSI were investigated. Among the important features of the results are (i) the observation of the vanishing of the FE state at 0 K and high pressure (> ∼ 9.5 kbar), and (ii) the observation of a new, pressure-induced effect in some samples of this material. The FE transition temperature Tc → 0 K with a finite slope dTc/dP, as we have observed for other displacive soft mode ferroelectrics. In these ferroelectrics the strictly harmonic frequency ω0 of the soft FE mode in the paraelectric phase is imaginary at 1 bar, and all of the stabilization of the mode frequency is provided by high-order anharmonicities. At sufficiently high pressure ω0 becomes real and the crystal becomes stable against the FE mode at all temperatures, i.e. the FE state vanishes. It is postulated that the new effect, which is manifested by a peak in the static susceptibility which grows rapidly with pressure, is associated with crystalline defects. Two other interesting aspects of the behavior of SbSI under high pressure are the possible existence of (i) a Curie critical (or tricritical) point, and (ii) a triple point. The available evidence for each is examined, and it is concluded that the question of the Curie critical point remains unsettled.