Crossover from propagon to diffuson thermal transport in SnTe due to nanodiamond inclusions leads to ultra-low lattice thermal conductivity

The “phonon-glass electron-crystal” is an infamously challenging thermoelectric material design principle due to the interconnectedness of thermal and electronic transport in materials. Here, the incorporation of ∼5-nm particles of diamond—a phonon crystal—into the thermoelectric matrix of SnTe is explored as a route toward low lattice thermal conductivity. This counterintuitive strategy works because the large acoustic property mismatch at the SnTe–diamond interface blocks thermal transport. Between 300 and 773 K, SnTe with 1.0 vol. % nanodiamond inclusion exhibits the lowest average and absolute lattice thermal conductivities of any reported SnTe material in this temperature range. The ultra-low lattice thermal conductivity of the nanocomposites is investigated in the two-channel framework—recently advanced in the context of glassy and disordered materials—whereby heat is transported by propagating and non-propagating phonons termed propagons and diffusons, respectively. Above ∼650 K, calculations demonstrate the breakdown of the phonon gas (propagon-only) model for describing the nanocomposite conductivity. At ∼773 K, conductivity reaches the glassy limit where thermal transport is mediated by diffusons. Neutron spectroscopy reveals that with the increase in temperature, phonon modes in SnTe broaden and overlap in energy. We propose that linewidth broadening from nanodiamond-induced scattering and Umklapp processes promotes coupling and wave-like tunneling between overlapping modes, thereby enhancing diffuson-mediated transport at the expense of propagon transport. This progression toward diffuson-dominated conduction represents a novel transport paradigm in primarily crystalline nanocomposites.