Ultralow thermal conductivity in the quaternary semiconducting chalcogenide Cs4[Ho26Cd7Se48] with an unprecedented closed cavity architecture

Abstract

Novel three-dimensional (3D) inorganic chalcogenide architectures have attracted much attention because of their rich structural assembly patterns and intriguing physical properties. Herein, four isostructural quaternary semiconducting chalcogenides, namely Cs₄[RE₂₆Cd₇Se₄₈] (RE = Ho, Er, Tm, and Lu), have been discovered and found to crystallize in a tetragonal system, space group I4₁/a (no. 88). Their structures feature 3D quasi-NaCl type architectures formed by edge-sharing and face-sharing of [RESe₆] and [MSe₆] (M = RE/Cd) octahedra, which are embedded with the unprecedented polyanionic [Cs₂@Se₁₈–Cd–Cs₂@Se₁₈] closed cavities. Significantly, the compound Cs₄[Ho₂₆Cd₇Se₄₈] exhibits ultralow thermal conductivity (0.38–0.29 W m⁻¹ K⁻¹) from 295 K to 675 K, which is one of the lowest values reported in crystalline chalcogenides. This result not only enriches the coordination chemistry of host–guest systems, but also suggests a new thought for the design of novel inorganic chalcogenides with intrinsic low thermal conductivities.