The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH 3NH 3) 2MnCl 4, from 10 to 300k

Abstract

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH 3NH 3) 2MnCl 4, has been measured over the range 10K < T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with Δ H t = 727±5 J mol −1 and Δ S t = 7.76±0.05 J K −1 mol −1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with Δ H t = 116±1J mol −1 and Δ S t = 0.451±0.004 J K −1mol −1. These results are discussed in the light of recent measurements on (CH 3NH 3) 2CdCl 4, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type. In addition to the structural phase transitions, (CH 3NH 3) 2MnCl 4 also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.