Heat capacity of a five-coordinated iron(III) complex with spin [formula omitted], bromobis ( N, N-dimethyldithiocarbamato) iron(III), between 0.4 and 300 K: New finding of a magnetic phase transition

Abstract

The heat capacity of bromobis( N, N-dimethyldithiocarbamato)iron(III) was measured between 0.4 and 300 K. A phase transition from a ferromagnetic to a paramagnetic state was found at T c = (0.837 ± 0.005) K, and a Schottky anomaly arising from the zero-field splitting was observed around 9 K. The total magnetic entropy and enthalpy, including the phase transition and the Schottky anomaly, were (11.74 ± 0.01) J K −1 mol −1 and (92.0 ± 0.4) J mol −1, respectively. The entropy was only 1.8% larger than R ln 4 ( = 11.53 J K −1 mol −1), confirming that the spin manifold is really a quartet (intermediate spin S = 3 2 ). The transition entropy and enthalpy were estimated to be (6.05 ±0.01) J K −1 mol −1 and (5.77 ± 0.01) J mol −1, respectively. The transition entropy was only 5% larger than R ln 2. Since the ground spin states of the present compound are characterized by a zero-field splitting of the S = 3 2 state into two Kramers' doublets, the phase transition is concerned with only the ground Kramers' doublet. The entropy and enthalpy above T c, which is a measure of the short-range order, are large but relatively small compared with those of the ethyl-homologues. This fact suggests that the magnetic lattice dimension in the present complex is somewhat different from the ethyl-homologues from a thermodynamic point of view. The magnitude of the exchange interaction determined from the magnetic transition temperature for both the methyl- and ethyl-homologues is monotonically increased in the order Cl, Br and I. This implies that the halogen atom plays an important role in the intermolecular exchange interaction.