Heat Capacities of Rb₂ZnBr₄ between 3 and 300 K—Thermodynamic Properties of Its Incommensurate Phase Transitions

抄録

The heat capacity of Rb₂ZnBr₄ was measured from 3 to 300 K. The commensurate-incommensurate phase transition (so-called lock-in transition) showed very small anomaly of the first-order at (193.6±0.3) K with ΔH_t=(9.3±1.0) J·mol⁻¹ and ΔS_t=(0.048±0.005) J·K⁻¹·mol⁻¹. Below the lock-in transition, two phase transitions were observed. A typical second-order anomaly was at (111.7±0.2) K with ΔH_t=(47±5) J·mol⁻¹ and ΔS_t=(0.45±0.05) J·K⁻¹·mol⁻¹. Another anomaly was observed at (76.5±0.1) K with ΔH_t=(2.0±0.3) J·mol⁻¹ and ΔS_t=(0.027±0.004) J·K⁻¹·mol⁻¹. A unified treatment of the successive transitions in K₂SeO₄, Rb₂ZnCl₄, and Rb₂ZnBr₄ is given based on a Landau-type phenomenological theory. Numerical calculation of heat capacities and entropies of the lock-in transitions showed general agreement with the experimental results.