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

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 E F 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).

Thermoelectric Zintl Compound In1−xGaxTe: Pure Acoustic Phonon Scattering and Dopant‐Induced Deformation Potential Reduction and Lattice Shrink

“…However, the Ga substitution introduces additional low‐frequency optical phonons and the low‐frequency peak locates at approximately 0.5 THz (Figure 9e). In this case, the impact of the low‐lying optical phonon modes on the κ l can be well reflected by fitting the experimental heat capacity ( C p ) data based on the Debye‐Einstein model [33] . The measured C p can be well fitted by the Debye–Einstein fitting with three Einstein oscillators, which corroborates the existence of low‐frequency optical phonons (Figure 9h).…”

Thermoelectric Zintl Compound In1−xGaxTe: Pure Acoustic Phonon Scattering and Dopant‐Induced Deformation Potential Reduction and Lattice Shrink

“…In this case, the impact of the low-lying optical phonon modes on the k l can be well reflected by fitting the experimental heat capacity (C p ) data based on the Debye-Einstein model. [33] The measured C p can be well fitted by the Debye-Einstein fitting with three Einstein oscillators, which corroborates the existence of low-frequency optical phonons (Figure 9h). The obtained three Einstein oscillator temperatures are Θ E1 = 51.9 K, Θ E2 = 96 K and Θ E3 = 122.6 K, which correspond to the Einstein oscillating frequencies of 0.46, 1.2 and 1.62 THz, respectively.…”

Transuranium Sulfide via the Boron Chalcogen Mixture Method and Reversible Water Uptake in the NaCuTS3 Family

“…We recently demonstrated a new convenient way to synthesize binary and complex uranium and thorium chalcogenides using stable ThO 2 and U 3 O 8 precursors. , The boron chalcogen mixture (BCM) method uses a mixture of elemental boron and chalcogen (sulfur, selenium, or tellurium) and utilizes boron as a sacrificial reagent to extract oxygen by forming the thermodynamically stable B 2 O 3 oxide, leaving thorium or uranium to react with the chalcogen (B 2 O 3 is subsequently removed by dissolution in water or methanol). The BCM method has been successfully used to convert many p-, and d-metal and lanthanide oxides to their chalcogenides, demonstrating that it can be successfully employed with nearly all metal oxides in the periodic table, except (so far) for transuranium elements, − for which the applicability of this method has not yet been established. The challenges in transuranium chalcogenides synthesis motivated us to test the applicability of the BCM method to neptunium containing chalcogenides by predicting the reaction outcome using computational thermodynamic data of the known binary actinides and by targeting the synthesis of a complex neptunium quaternary chalcogenide.…”