Inorganic Crystals with Glass-Like and Ultralow Thermal Conductivities

The ability to control the transport of thermal energy is critical in a wide variety of technologies. At the same time, understanding the underlying microscopic mechanisms of thermal transport in solids continues to be a central goal of condensed matter and materials physics, with many persistent challenges and unanswered questions. One of the remarkable findings has been the observation that some crystalline materials have very low, glass-like thermal conductivities despite long-range order in the arrangement of the atoms in their structure. Although examples with such unusual behavior were initially rare, the number of crystalline materials known to have glass-like thermal conductivities has grown significantly in the past two decades. Moreover, some fully dense inorganic solids have recently been discovered that possess ultralow thermal conductivities below the so-called glass limit. In this review, we use several specific examples to highlight the salient structural and lattice dynamical features of these intriguing glass-like crystals, focusing on current understanding of the microscopic mechanisms that cause these crystals to conduct heat like a glass. The study of such materials continues to push our understanding of heat transport in solids and the roles that chemical bonding and structural order and disorder play in thermal transport processes.