Mixed-Valence CsCu₄Se₃: Large Phonon Anharmonicity Driven by the Hierarchy of the Rigid [(Cu⁺)₄(Se^2–)₂](Se^–) Double Anti-CaF₂ Layer and the Soft Cs⁺ Sublattice

Crystalline solids that exhibit inherently low lattice thermal conductivity (κ_lat) have attracted a great deal of attention because they offer the only independent control for pursuing a high thermoelectric figure of merit (ZT). Herein, we report the successful preparation of CsCu₄Q₃ (Q = S (compound 1), Se (compound 2)) with the aid of a safe and facile boron–chalcogen method. The single-crystal diffraction data confirm the P4/mmm hierarchical structures built up by the mixed-valence [(Cu⁺)₄(Q^2–)₂]­(Q^–) double anti-CaF₂ layer and the NaCl-type Cs⁺ sublattice involving multiple bonding interactions. The electron-poor compound CsCu₄Q₃ features Cu–Q antibonding states around E_F that facilitates a high σ value of 3100 S/cm in 2 at 323 K. Significantly, the ultralow κ_lat value of 2, 0.20 W/m/K at 650 K (70% lower than that of Cu₂Se), is mainly driven by the vibrational coupling of the rigid double anti-CaF₂ 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 2 drops sharply from 118 cm^–1 (of Cu₂Se) to 48 cm^–1. 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 Cu₂Se (2320 m/s; 292 K).