Thermodynamic study of commensurate-incommensurate phase transitions in Rb2ZnCl4

We have performed simultaneous measurements of heat capacity and dielectric constant on a high-quality single crystal of Rb2ZnCl4 near the commensurat e-incommensurate transition area. Maximum heat capacity is found to be at a higher temperature than the maximum dielectric constant, both for heating and cooling and in the area of a jump-wise change of the refractive index f ound earlier. When measuring heat capacity a peak-type anomaly was detected and the enthalpy of transition was measured. We compare our heat-capacity data with those for the powder samples and both sets with a phenomenologica l theory. We concluded that the Curie law for heat capacity predicted by th e theory is observed in both cases with reasonable values for the Curie con stant. We were unable to discriminate latent heat as in previous experiment s with the powder sample; however, we estimated an upper limit for the late nt heat by subtracting the contribution of specific heat to the enthalpy tr ansition. The latent-heat value estimated both from the Clapeyron-Clausius formula and the phenomenological theory is qualitatively in accordance with the upper limit of this value found in our results. [S0163-1829(99)06321-3 ].