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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
- Source :
- Journal of the American Chemical Society. 143:18490-18501
- Publication Year :
- 2021
- Publisher :
- American Chemical Society (ACS), 2021.
-
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).
Details
- ISSN :
- 15205126 and 00027863
- Volume :
- 143
- Database :
- OpenAIRE
- Journal :
- Journal of the American Chemical Society
- Accession number :
- edsair.doi...........d47f2de0622b3e57dcc5aea0761ba3ae