"High-frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals", B. Sun, S. Niu, N. Shulumba, K. L. Page, K. Mahalingam, J. Milam-Guerrero, R. Haiges, M. Mecklenburg, B. C. Melot, Y. D. Jho, B. M. Howe, M. E. Manley, J. Ravichandran, and A. J. Minnich, Nature Communications 11, 6039 (2020)
Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In the phonon picture of thermal transport applicable to most crystals, thermal conductivity decreases with increasing temperature above the Debye temperature owing to increasing phonon-phonon scattering rates. Defect-free crystals that exhibit a glass-like trend of low thermal conductivity that increases with temperature, refecting a breakdown in this phonon picture,
are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this advantageous behavior is rare and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in all-inorganic hexagonal perovskite chalcogenide single crystals, BaTiS3 and Sr8=7TiS3, despite their symmetric and relatively simple primitive cell. Temperature-dependent measurements of the atomic pair distribution function using neutron diraction reveals marked strengthening of positional disorder in the S and Ti atoms at cryogenic temperatures that disrupts the apparent periodicity of the crystal. Our study identies a new symmetry-breaking mechanism for intrinsically low thermal conductivity that results from asymmetric but soft and highly polarizable lattice, providing guidance for the design of materials with intrinsically low thermal conductivity.