Specific heat anomalies near 147 K for K2ZnCl4

Single crystals of K2ZnCl4 with high purity mere gown by the Czochralski method in order to avoid the strong influence of water remaining in the crystal. The heat capacity for a single crystal was measured for cooling as well as heating runs, using a home-built adiabatic calorimeter with a temperature precision of 1 mK. The existence of a lo low-temperature incommensurate phase was revealed in these measurements for the first time. On heating, two sharp peaks were found at 146.983 +/- 0.002 K (peak I) and at 146.956 +/- 0.002 K (peak II), and the temperature interval between the two peaks amounted to approximately 0.027 K. These peaks showed a thermal hysteresis of about 0.1 K. The shapes of peak II obtained on both heating and cooling measurement are characteristic of a commensurate - incommensurate phase transition.