We have performed simultaneous measurements of heat capacity and dielectric constant on a high-quality single crystal of ${\mathrm{Rb}}_2{\mathrm{ZnCl}}_4$ near the commensurate-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 found 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 phenomenological theory. We concluded that the Curie law for heat capacity predicted by the theory is observed in both cases with reasonable values for the Curie constant. We were unable to discriminate latent heat as in previous experiments with the powder sample; however, we estimated an upper limit for the latent heat by subtracting the contribution of specific heat to the enthalpy transition. 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.

• Received 23 July 1998

DOI:https://doi.org/10.1103/PhysRevB.59.14265