Mixed-Valence CsCu4Se3: Large Phonon Anharmonicity Driven by the Hierarchy of the Rigid [(Cu+)(4)(Se2-)(2)](Se-) Double Anti-CaF2 Layer and the Soft Cs+ Sublattice

Crystalline solids that exhibit inherently low lattice thermal conductivity (kappa(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 CsCu(4)Q(3) (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 P-4/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 CsCu(4)Q(3) features Cu-Q antibonding states around E-F that facilitates a high sigma value of 3100 S/cm in 2 at 323 K. Significantly, the ultralow kappa(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).