The heat capacity of the layer compounds (CH 3CH 2CH 2NH 3) 2MnCl 4 and (CH 3CH 2CH 2NH 3) 2CdCl 4 from 10 K TO 300 K

Abstract

The heat capacity of the layer compounds tetrachlorobis ( n-propylammonium) manganese II and tetrachlorobis ( n-propylammonium) cadmium II, (CH 3CH 2CH 2NH 3) 2MnCl 4 and (CH 3CH 2CH 2NH 3) 2CdCl 4 respectively, has been measured over the temperature range 10 K ≲ T ≲ 300 K. Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (Δ H t = 586 ± 2 J mol −1; Δ S t = 5.47 ± 0.02 J K −1mol −1) and at T =164.3 ± (Δ H t = 496 ± 7 J mol −1; Δ S t =3.29 ± 0.05 J K −1mol −1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain. A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (Δ H t = 1472.3 ± 0.1 J mol −1; Δ S t = 13.956 ± 0.001 J K −1mol −1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions( T = 178.7 ± 0.3 K; Δ H t = 982 ± 4 J mol −1 Δ S t = 6.16 ± 0.02 J K − mol −1) is known to be between two orthorhombic structures, while the structural changes at the lower transition ( T= 156.8 ± 0.2 K; Δ H t = 598 ± 5 J mol −1, Δ S t = 3.85 ± 0.03 J K −1 mol −1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds. In addition to the structural phase transitions (CH 3CH 2CH 2NH 3) 2MnCl 4 magnetically orders at t≲ 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.