Mixed-Valence CsCuSe: Large Phonon Anharmonicity Driven by the Hierarchy of the Rigid [(Cu)(Se)](Se) Double Anti-CaF Layer and the Soft Cs Sublattice.

Crystalline solids that exhibit inherently low lattice thermal conductivity (κ) have attracted a great deal of attention because they offer the only independent control for pursuing a high thermoelectric figure of merit (). Herein, we report the successful preparation of CsCuQ (Q = S (compound ), Se (compound )) with the aid of a safe and facile boron-chalcogen method. The single-crystal diffraction data confirm the 4/ hierarchical structures built up by the mixed-valence [(Cu)(Q)](Q) double anti-CaF layer and the NaCl-type Cs sublattice involving multiple bonding interactions. The electron-poor compound CsCuQ features Cu-Q antibonding states around that facilitates a high σ value of 3100 S/cm in at 323 K. Significantly, the ultralow κ value of , 0.20 W/m/K at 650 K (70% lower than that of CuSe), is mainly driven by the vibrational coupling of the rigid double anti-CaF layer and the soft NaCl-type sublattice. The hierarchical structure increases the bond multiplicity, which eventually leads to a large phonon anharmonicity, as evidenced by the effective scattering of the low-lying optical phonons to the heat-carrying acoustic phonons. Consequently, the acoustic phonon frequency in drops sharply from 118 cm (of CuSe) to 48 cm. In addition, the elastic properties indicate that the hierarchical structure largely inhibits the transverse phonon modes, leading to a sound velocity (1571 m/s) and a Debye temperature (189 K) lower than those of CuSe (2320 m/s; 292 K).