Two-channel model for ultralow thermal conductivity of crystalline Tl3VSe4

Glass-like and crystal-likeCrystals with glass-like ultralow thermal conductivity are appealing as barrier coatings and thermoelectric materials. Mukhopadhyay et al. developed a class of thallium selenides with glass-like thermal conductivity. These materials may be promising for applications, but they also require the combination of glass-like and crystal-like thermal transport to explain their thermal properties. This two-channel model can be used to identify potential ultralow-thermal-conductivity compounds.Science, this issue p. 1455Solids with ultralow thermal conductivity are of great interest as thermal barrier coatings for insulation or thermoelectrics for energy conversion. However, the theoretical limits of lattice thermal conductivity (κ) are unclear. In typical crystals a phonon picture is valid, whereas lowest κ values occur in highly disordered materials where this picture fails and heat is supposedly carried by random walk among uncorrelated oscillators. Here we identify a simple crystal, Tl3VSe4, with a calculated phonon κ [0.16 Watts per meter-Kelvin (W/m-K)] one-half that of our measured κ (0.30 W/m-K) at 300 K, approaching disorder κ values, although Raman spectra, specific heat, and temperature dependence of κ reveal typical phonon characteristics. Adding a transport component based on uncorrelated oscillators explains the measured κ and suggests that a two-channel model is necessary for crystals with ultralow κ.