Cu₄Bi₄Se₉: A Thermoelectric Symphony of Rattling, Anharmonic Lone-pair, and Structural Complexity

Abstract

Thermoelectric technology, enabling an environmentally friendly direct heat to electricity conversion, provides a possible alternative energy solution. To obtain a higher thermoelectric conversion efficiency, a larger dimensionless figure of merit ZT is required, which is, however, very difficult owing to the mutually restricted and even reversely correlated key property parameters. Herein, we report for the first time the thermoelectric properties of novel Cu 4 Bi 4 S 9 and Cu 4 Bi 4 Se 9 materials with complicated orthorhombic Pnma structures. Cu 4 Bi 4 Se 9 exhibits an extremely low lattice thermal conductivity of about 0.29−0.35 W m −1 K −1 that is mainly ascribed to the high lattice anharmonicity coming from the synergistic effect of the crystal structure complexity, soft Cu−Se bonds with lower bonding energy, rattling of the Cu atoms, and the high anharmonicity of Bi atoms carrying stereochemically active lone-pair electrons. In spite of its poor electrical conductivity of 7.33 S cm −1 , Cu 4 Bi 4 Se 9 realizes a power factor of about 1.37 μW cm −1 K −2 at ∼530 K, and a figure of merit, ZT ∼0.24 at ∼530 K. Such a value is comparable with those of the Cu/Ag-Bi/Sb-S/Se-based ternary compounds, particularly, the 19 times higher ZT improvement with respect to the isostructural Cu 4 Bi 4 S 9 suggests that the enhancing factors mentioned above play significant roles.

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

“…As shown in Figure a,b, the relatively small band gap is an indication of a striking electrical transport property. The measured band gaps of 2 (0.94 eV) and 1 (1.88 eV) agree with the DFT calculations, which are also consistent with those of Cu 4 Bi 4 Se 9 (0.45 eV) and Cu 4 Bi 4 S 9 (0.81 eV) . The electronic structures also depict slightly flattened bands near the Fermi level, indicating a considerably large band effective mass ( m *) (Figure a,b).…”

“…The lattice thermal conductivity (κ lat ) is the only independent parameter to enhance the ZT . Ongoing efforts in phonon blocking engineering, e.g., all-scale hierarchical architectures (point defects, endotaxial precipitates, grain boundaries), − solid solution alloying, , and lone-pair anharmonicity, − have been proven to effectively reduce κ lat . However, these strategies face the challenge of boosting ZT ; that is, detrimental charge carrier scattering decreases the electrical mobility as well as the repeatability and stability of the materials, which are major concerns in industry.…”

“…Thus, intrinsic ultralow-κ lat solids are important because they carry independent control among transport carriers and reveal the correlation among bonding nature, lattice dynamics, and heat propagation. Ultimately, the structural chemistry of a solid requires a comprehensive understanding of the valley degeneracy ( N V ), density of states effective mass ( m DOS * = N V 2/3 m band *), , band gap ( E g ), carrier density ( n ), and mobility (μ), as well as the phonon velocity ( v ) and phonon scattering, i.e., relaxation time (τ), Grüneisen parameter (γ), average atomic mass ( M ), phonon frequency (ω), and cell volume ( V ), to finally obtain a high-performance TE material. For instance, Cu 2 Se shows a crystal structure of highly disordered Cu ions (liquidlike cation dynamics) that causes strong phonon scattering by suppressing the transverse phonon modes and thus lowers κ lat .…”

Quasi-layered Crystal Structure Coupled with Point Defects Leading to Ultralow Lattice Thermal Conductivity in n-Type Cu2.83Bi10Se16

“…There are seven phases, CuBiSe 2 (two modifications), , Cu 1.6 Bi 4.8 Se 8 , Cu 1.78 Bi 4.73 Se 8 , Cu 3 BiSe 3 , Cu 6 BiSe 6 , and Cu 4 Bi 4 Se 9 reported in the ternary Cu–Bi–Se system, which feature complex crystal structures. The research on Cu 4 Bi 4 Se 9 demonstrated its low thermal conductivity and good thermoelectric performance originating from its structural features.…”

A material catalogue with glass-like thermal conductivity mediated by crystallographic occupancy for thermoelectric application

“…The importance of partial occupancy as a function of unit cell volume and average mass per atom. (a) The obtained database of potential candidates of compounds with partial occupancy; (b) the comparison of κlat among common semiconductors (including elemental, I-V-VI2, I2-IV-VI3, I3-V-VI4, I-V-VI2, II2-IV, III-V, IV-VI, V3-VI2 compounds)61 , Zintl compounds62,63 (including CoSb3, CaZn2Sb2, YbZn2Sb2, Ca3AlSb3, Sr3GaSb3,Yb11InSb9, and Yb14MnSb9) and compounds with partial occupancy (including Ag8GeTe6,64 Ba8Ga16Ge30,65 Ba2Sb2Se5,66 Cu4Bi4Se9,67 Cu5FeS4,68 Cu4Sn7S16,69 Cu2SnSe4,70 Ge1Sb4Te7, 71 Zn8Sb7 72 as well as Cu6Te3S, Bi5CuS8 and Cr5Te8). Only some important compounds are named in Figure7b.…”