Reversible Regulation of Thermal Conductivity with Spin State Transitions

The capability of regulating thermal transport in solids enables efficient thermal energy storage and heat management. Thermal conductivity in materials with metal-insulator transitions or structural phase transitions undergoes abrupt changes across the transition temperature, yet the thermal conductivity switching ratio is limited. Here we report a new mechanism for regulation of thermal conductivity using spin state transition. We observe a significant drop in thermal conductivity exceeding a factor of 2 in an iron molecular complex upon surpassing its spin state transition temperature, measured by frequency-domain thermoreflectance. As temperature rises, the singlet-to-quintet molecular spin state transition occurs when the ligand field splitting energy becomes smaller than the pairing energy. The occupation of antibonding orbitals weakens both metal-ligand bonds (intramolecular) and hydrogen bonds (intermolecular), leading to lower group velocities of heat-carrying phonons evident from inelastic neutron scattering. In addition to the large thermal conductivity contrast, the minimal volume expansion and high reversibility position the spin state transition an effective mechanism for the temperature regulation of thermal transport.