Calorimetric measurements of the phase transition in Rochelle salt–ammonium Rochelle salt mixed crystals

Abstract

The specific heats, which are given with absolute value, of Rochelle salt-ammonium Rochelle salt mixed crystals (RS 1Ϫx -ARS x ͒ in a region 0.18рxр0.9 have been measured between about 10 K and 300 K by an ac calorimetric method. A very small anomaly was found around the transition for any x. It becomes larger as x increases. The anomalies have been well described using the Landau-type free energy expanded to the sixth order of polarization. It was found that, as x increases, the transition point approaches the tricritical point at which a crossover from the second-order phase transition to the first-order one takes place. Transition entropy has been calculated from the anomalous specific heat. It increases from about 8% to about 55% of Rln2 with increasing x in the measured region. Very small values for small x in this region suggest the displacivelike character in the phase transition. And the order-disorder motion contributes more and more to the phase transition with increasing x.

Theory of Rochelle Salt: Beyond the Mitsui Model

“…Such displacements can be interpreted also as changes in the population ratio of two sites in the disordered paraelectric structure (revealed in the structure investigations [8,9]). Large values of the anisotropic temperature factors can be also connected with local disorder [10].…”

Piezoelectric Effect in Rochelle Salt

“…Respective static displacements initiate the emergence of dipole moments in local structure units in ferroelectric phase. Such displacements can be interpreted also as changes in the population ratio of two equilibrium positions of sites in the paraelectric structure (revealed in the structure studies (Noda et al, 2000;Shiozaki et al, 2001)). The order-disorder pattern of phase transitions in RS forms the basis of the semimicroscopic Mitsui model (Mitsui, 1958).…”

Study of Dielectric Constant and Loss Tangent of Rochelle Salt Crystal

“…Shiozaki et al 12 have studied structural changes in Rochelle salt due to disordering. Noda et al 13 have studied calorimetric measurements of phase transition in ammonium mixed Rochelle say. Kikuta et al 14 have also studied mixed Rochelle salt crystals.…”