Heat Capacity and Magnetic Phase Transition of the Molecule-Based Magnet
TOT• + · FeIII Br4−

Heat capacities of 2,2′:6′,2″:6″,6-trioxytriphenylamine radical cation TOT• + (S = 1/2) and FeIII Br4− (S = 5/2) salt TOT• + · FeIII Br4− were measured by relaxation method under magnetic fields. A heat capacity peak was observed at 7.30 K. This peak is due to antiferromagnetic phase transition from the magnetic field dependence of the peak temperature. Magnetic entropy was evaluated to be 14.4 J K−1 mol−1, which is close to Rln6 (= 14.9 J K−1 mol−1) for S = 5/2 spin systems. From the ratio of the magnetic entropy above the magnetic phase transition temperature to the whole magnetic entropy, TOT• + · Fe Br4− would have a three-dimensional body-centered cubic magnetic structure with J/kB = −0.39 K, which agrees roughly with the value J/kB = −0.16 K derived by the mean-field approximation with the magnetic phase transition temperature. Magnetic interaction within the Fe Br4− dimer was estimated to be Jd/kB = −1.1 K.

(by X.-Z. Lan & Y. Miyazaki)

	Fig. 1. Molecular structures of DOT• + (left) and TOT• + (right).
	Fig. 2. Heat capacities of TOT• + · Fe Br4− under magnetic fields. Solid curve indicates the lattice heat capacity. For the sake of clarity, the heat capacities except for the zero-field heat capacity are shifted upwards.
	Fig. 3. Magnetic heat capacities of TOT• + · Fe Br4− under magnetic fields. For the sake of clarity, the magnetic heat capacities except for the zero-field magnetic heat capacity are shifted upwards.