Two-channel model for ultralow thermal conductivity of crystalline Tl₃VSe₄

Solids 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 (k) are unclear. In typical crystals a phonon picture is valid, whereas lowest k 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 k [0.16 Watts per meter-Kelvin (W/m-K)] one-half that of our measured k (0.30 W/m-K) at 300 K, approaching disorder k values, although Raman spectra, specific heat, and temperature dependence of k reveal typical phonon characteristics. Adding a transport component based on uncorrelated oscillators explains the measured k and suggests that a two-channel model is necessary for crystals with ultralow k.