Reversible Regulation of Thermal Conductivity through Spin-Crossover Transitions

The development of strategies to modulate the thermal conductivity of a solid in response to an external stimulus is critical to the creation of high-performance thermal regulators, thermal switches, and thermal diodes─devices which would be enabling for a wide range of emerging technologies. Here, we report a new mechanism for achieving switchable solid-state thermal conductivity through a first-order spin-crossover phase transition. Specifically, we show that single crystals of the molecular spin-crossover complex Fe(HB(tz)3)2 [HB(tz)3- = hydrotris(1,2,4-triazol-1-yl)borate] exhibit a large drop in thermal conductivity, more than 4-fold, across an electronic spin transition. This thermal conductivity change is highly reversible and can be attributed to lower group velocities of heat-carrying phonons and increased phonon scattering in the high-spin phase of the compound as a result of weaker metal-ligand bonds. We further demonstrate the generalizability of this phenomenon by showing a similarly large change in thermal conductivity for another Fe(II) spin-crossover material with a different coordination environment and transition temperature. Owing to the large structural and chemical diversity of spin-crossover materials and the rich variety of stimuli that can induce electronic spin transitions, these results establish a powerful approach to manipulating thermal transport within solid materials.