Polymorphism in (n-C7H15NH3)2MnCl4 and (n-C7H15NH3)2CdCl4 as determined by heat-capacity measurements from 10 to 325 K

Abstract

The heat capacities of the layer compounds (n-C₇H₁₅NH₃)₂MnCl₄ and (n-C₇H₁₅NH₃)₂CdCl₄ have been measured by adiabatic calorimetry over the temperature range from 10 to 325 K. Two structural phase transitions were observed in each compound, at T= 247.95 ± 0.05 K (ΔH_trans= 16 930 ± 30 J mol^–1, ΔS_trans= 68.2 ± 0.1 J K^–1 mol^–1) and at T= 313.8 ± 0.1 K (ΔH_trans= 10 197 ± 9 J mol^–1, ΔS_trans= 32.5 ± 0.1 J K^–1 mol^–1) in the Mn compound, and at T= 250.00 ± 0.05 K (ΔH_trans= 17 630 ± 10 J mol^–1, ΔS_trans= 71.38 ± 0.04 J K^–1 mol^–1) and at T= 316.74 ± 0.05 K (ΔH_trans= 5060 ± 80 J mol^–1 and ΔS_trans= 16.0 ± 0.2 J K^–1 mol^–1) in the Cd compound. The nature of these transitions is discussed in terms of those found in other compounds of the same series with shorter and longer alkyl chains. In addition, the magnetic contribution to the heat capacity of the Mn compound is shown to be consistent with that of a quasi-two-dimensional Heisenberg antiferromagnet.