Heat capacity and thermodynamic functions of CsCrCl3 and RbCrCl3. Structural phase transitions

We present the heat capacities measured by adiabatic calorimetry from 6 to 350 K. and by differential scanning calorimetry from 300 to 500 K, of CsCrCl3 and RbCrCl3. A first-order transition at Tsub(c) = (171.1 +- 0.1) K was detected for CsCrCl3. The RbCrCl3- showed at Tsub(c) = (193.3 +- 0.1) K a transition with thermal hysteresis at temperatures just below the maximum. At T1 = (440 +- 10) K a continuous transition was also detected. Furthermore, at Tsub(N) approx. 16 K, and for both compounds, a small bump due to magnetic long-range ordering was observed. The thermodynamic functions at 298.15 K are presented. The distortion of the (CrCl6)4- complexes is explained in terms of a strong Jahn-Teller coupling scheme. The structural phase transitions are discussed as cooperative distortions caused by elastic interactions among the complexes. The character of the various transitions is analysed in terms of Landau's theory. (author)