Mixed-Valence CsCu<sub>4</sub>Se<sub>3</sub>: Large Phonon Anharmonicity Driven by the Hierarchy of the Rigid [(Cu<sup>+</sup>)<sub>4</sub>(Se<sup>2–</sup>)<sub>2</sub>](Se<sup>–</sup>) Double Anti-CaF<sub>2</sub> Layer and the Soft Cs<sup>+</sup> Sublattice
Ni Ma, Fan Li, Jian-Gao Li, Xin Liu, Dong‐Bo Zhang, Yanyan Li, Ling Chen, Li‐Ming Wu
Abstract
Crystalline solids that exhibit inherently low lattice thermal conductivity (κlat) 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(Q2–)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 EF that facilitates a high σ value of 3100 S/cm in 2 at 323 K. Significantly, the ultralow κlat 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).