Microstructural design of a new thermoelectric material system for Shanghai Silicate has been advanced

In the realm of non-cubic chalcopyrite structural compounds, a novel "pseudo-cubic" structure design approach has been developed to achieve highly degenerate energy bands similar to those found in cubic structures. This advancement leads to improved electrical transport and thermoelectric performance. (a) A schematic of the crystal and energy band structure of a cubic sphalerite structure is shown. (b) The crystal and energy band structure of a ternary chalcopyrite compound are illustrated. The "cubic" structure comprises a cubic cation framework and a non-cubic, twisted anionic lattice. (c) The relationship between the ZT value and the valence band top splitting value ΔCF is presented. Red dots represent high-temperature compounds with ZT > 1, while blue and green dots correspond to (Ag,Cu)InTe₂ and Cu(In,Ga)Te₂ solid solutions, respectively.

Figure 2 illustrates the Unity-η screening criteria. The red dots represent high-temperature compounds with ZT > 1, while the pink band marks the η ≈ 1 region, which could be a potential area for discovering new thermoelectric materials.

Figure 3 shows the design of η vs. a and ΔCF vs. a for solid solution compounds. The red star indicates two compounds with η < 1 (ΔCF < 0) and η > 1 (ΔCF > 0). The resulting solid solution is plotted with a red dotted line representing the trend.

Fig. 4 presents the thermoelectric performance of CuInTe₂ and AgInTe₂ (CuGaTe₂) and their solid solutions. Developing high-performance thermoelectric materials and expanding thermoelectric conversion technology remain key goals in the field. The performance of these materials is typically evaluated using the dimensionless figure of merit, ZT, which depends on intrinsic properties such as the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature. Historically, research has focused on limited material systems like SiGe, PbTe, skutterudite, Half-Heusler, Mg₂Si, and Cu₂Se. These materials often exhibit highly symmetrical cubic structures, which lead to degenerate or multi-valley energy bands at the top of the valence band or bottom of the conduction band, resulting in good electrical transport. However, many naturally occurring compounds have non-cubic structures, offering low bandgaps and low thermal conductivity but have been overlooked by researchers. The diamond-like structure, derived from a diamond lattice with tetrahedral bonding, can be distorted due to differences in atomic radii and chemical valence, leading to lower thermal conductivity. By adjusting the composition, the bandgap can be tuned across a wide range, making these materials promising for thermoelectric applications. Earlier studies by the Shanghai Institute of Ceramics reported that CuInTe₂ achieved a ZT of 1.18 at 850 K, and later, CuGaTe₂ reached a ZT of 1.4. Despite this, the mechanism behind high thermoelectric performance in non-cubic compounds remained unclear, limiting further exploration. Recently, researchers from the Shanghai Institute of Ceramics, in collaboration with international teams, introduced the "pseudo-cubic" microstructure design concept. This method involves forming a near-cubic cation framework to enhance electrical transport, while distorting anion lattices to reduce thermal conductivity. This balanced approach enables a high ZT value. Through theoretical calculations and experiments, they identified the Unity-η rule, where η = c/2a ≈ 1 correlates with minimal valence band splitting (ΔCF ≈ 0), leading to optimal performance. High-performing materials like CuInTe₂ and CuGaTe₂ align with this rule. For compounds not meeting the Unity-η condition, a solid solution design principle was proposed. By combining materials with complementary η values, a solid solution with η ≈ 1 and ΔCF ≈ 0 can be formed, enhancing thermoelectric performance. Experimental validation confirmed improved ZT values in CuInTe₂ and AgInTe₂ solid solutions. This "pseudo-cubic" design offers a new strategy for exploring high-performance non-cubic thermoelectric materials, opening up exciting possibilities for future research and industrial applications.

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