Specific heat measurements at low temperatures in K2MCl6 antifluorite crystals

Abstract

Measurements of the specific heat at low temperatures are reported for single-crystal samples of K2ReCl6, K2IrCl6, and K2PtCl6. The data for K2IrCl6 give strong evidence that in addition to the well-established antiferromagnetic phase transition occurring at a temperature T N = 3.01 ± 0.01 K, a displacive phase transition occurs at T c= 2.83 ± 0.02 K. Although the compounds are structurally similar, the background specific heats at low temperatures are significantly different. A careful analysis of these data plus previous data for K2OsCl6 shows that the reason for the large differences in the lattice specific heats is the presence of low-lying rotary modes. A Debye temperature of 91.1 ± 0.5 K is deduced and effective rotary mode frequencies of 28.6, 29.1, 30.0, and 37.9 cm−1 are found for K2ReCl6, K2OsCl6, K2IrCl6, and K2PtCl6, respectively.