The strong correlations between thermal conductivities and electronic spin states in the crystals of Fe(III) spin...

Solids that change their thermal conductivity during a phase transition can be useful in the development of a thermal switch to allow control of heat flow and reduce energy consumption. Although a crystal of a spin crossover (SCO) complex is a representative solid with spin states correlated with heat transporting lattice vibrations, the heat transporting property of a crystal of the SCO complex during a spin state transition has not yet been reported. In this work, we report that the temperature dependence of the thermal conductivity of mononuclear Fe(III) SCO complexes is greatly affected by spin state transitions. It was found that the thermal conductivity was minimized at temperatures near the beginning edge of the spin state transitions, and the product of the velocity and the mean free path of phonons also reached a minimum close to the temperature at which the spin state transition progressed by 50%. These findings suggest that the spin state transitions accompanying the coordination bond length elongation and lowering of the vibration energy are allowed at a temperature where the mean-free path of phonons is minimized to the extent of intermolecular distances. These findings also indicate that SCO complexes reported in the literature are promising candidates for heat transportation switch materials.