Phase Transition Behavior of a Manganese(III) Spin-Crossover Complex Examined through Dielectric and Magnetic Responses

摘要

The octahedral manganese(III) complex, (H_(3)taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine), has long been known as a rare example of (3d)~(4) spin-crossover system, having a steep spin-crossover phase transition at T _(c) = 48 K between a high-spin ~(5)E_(g) and a low-spin ~(3)T_(2g) states. Since the high-spin state is obviously subject to the Jahn-Teller effect, the electronic states are coupled with lower-symmetry deformation modes lifting ~(5)E_(g) orbital degeneracy in addition to a symmetrical breathing mode responsible for the spin crossover. This simultaneous involvement of two molecular bistabilities may afford a variety of condensed states (phases) due to competitive or cooperative interactions. The phase diagram and the phase transition behavior were thoroughly explored via several probes, including dielectric measurements, magnetization under pulsed high field, HF-EPR, and magnetic susceptibility measurements using high-pressure clamp cell. All the results were successfully interpreted in a unified fashion based on a four-state Ising-Potts model with mean-field approximation.