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

Heat capacities of two crystal polymorphs (crystals A and B) of 2-(nitronyl nitroxide)-9,10-dipheny-9,10-dihydrolphenazine radical cation NNDPP• + (S = 1) and FeIII Br4− (S = 5/2) salt NNDPP• + · FeIII Br4− were measured by relaxation method under magnetic fields. Crystals A and B exhibited heat capacity peaks due to magnetic phase transition at 3.38 K and 6.74 K, respectively. From the magnetic field dependences of these magnetic phase transition, crystal A is antiferromagnetic and crystal B is ferromagnetic. The magnetic entropies of crystals A and B were evaluated to be 24.0 J K−1 mol−1 and 23.8 J K−1 mol−1, respectively, which are very close to Rln(3×6) (= 24.0 J K−1 mol−1) for S = 1 and S = 5/2 spin systems. The zero-field magnetic heat capacities of crystals A and B above the magnetic phase transition temperatures are expressed well by the theoretical heat capacity curve for the high-temperature expansion of an S = 3/2 one-dimensional antiferromagnetic Heisenberg model with the intrachain magnetic interaction J/kB = −2.0 K and that of an S = 5/2 one-dimensional antiferromagnetic Heisenberg model with the intrachain magnetic interaction J/kB = −1.0 K, respectively. From the mean-field approximation by use of the derived intrachain magnetic interactions and the magnetic phase transition temperatures, the interchain magnetic interactions for both crystals were estimated to be |zJ′/kB| ≈ 0.28 K.

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

Fig. 1. Molecular structure of NNDPP• +.

Fig. 2. Magnetic heat capacities of NNDPP• + · Fe Br4− (crystals A (top) and B (bottom)) under magnetic fields. For the sake of clarity, the magnetic heat capacities except for the zero-field magnetic heat capacities are shifted upwards. Solid curves indicate the theoretical heat capacity for high-temperature expansion of S = 3/2 one-dimensional antiferromagnetic Heisenberg model with J/kB = −2.0 K for crystal A and that of S = 5/2 one-dimensional antiferromagnetic Heisenberg model with J/kB = −1.0 K for crystal B, respectively.