Research Background:
Against the backdrop of ongoing advancements in traditional thermoelectric materials (e.g., Bi₂Te₃, PbTe), binary indium chalcogenides (In-X, X = Te/Se/S) are emerging as a new research focus due to their unique structural properties and excellent thermoelectric performance. These materials form diverse compound systems, such as In₄Te₃, InTe, In₃Te₄, In₂Te₃, In₂Te₅, In₄Se₃, InSe, In₆Se₇, In₃Se₄, In₂Se₃, In₂Se₃, InS, In₆S₇, In₃S₄, and In₂S₃, through varying stoichiometries of indium and chalcogen elements. They exhibit rich ionic/covalent mixed bonding and multivalent characteristics (e.g., coexistence of In⁺ and In³⁺ in InTe). Their intrinsic thermal conductivity is below 2 W m^-1 K^-1 (@300 K), with a maximum thermoelectric figure of merit zT exceeding 0.5, particularly in In-Te and In-Se systems, as shown in Figure 1. Beyond thermoelectric applications, these materials also hold significant value in optoelectronic devices and ultrafast lasers due to their excellent nonlinear effects, high damage thresholds, and ideal bandgaps, offering new avenues for developing next-generation energy conversion materials.

Key Point 1: Crystal Structural Features
Binary indium chalcogenides not only exhibit traditional In-X bonding but also feature unique structural motifs such as In-In metallic bonds, chalcogen dimer bonds (Se-Se/Te-Te), and In3 atomic chains, as illustrated in Figure 2. Additionally, the mixed valence states of indium (e.g., In⁺, In²⁺, and In³⁺) contribute significantly to structural stability, electrical and thermal transport properties. The interplay between different bonding types and valence states leads to diverse and complex behaviors, making these materials highly valuable for research and applications, while providing insights for designing novel thermoelectric materials.

Key Point 2: Microscopic Regulation Mechanisms of Unconventional Bonding on Electron-Phonon Transport
The review employs systematic first-principles calculations to reveal the unique regulatory effects of unconventional chemical bonds on electron-phonon transport in binary indium chalcogenides. The study finds that In-In metallic bonds and chalcogen dimer bonds (Se-Se/Te-Te) not only significantly influence electronic structures near the Fermi level (Figure 3), but also introduce low-frequency optical phonon branches and enhance phonon-phonon scattering to achieve ultralow lattice thermal conductivity. Notably, the “rattling” vibration modes of isolated In⁺ ions substantially increase anharmonicity, while phonon softening due to unconventional bonding further suppresses thermal transport. These microscopic mechanisms are validated in systems like In₄Se₃ and InTe (Figure 4), offering new theoretical perspectives for understanding “electron-phonon decoupling”.

Key Point 3: Multidimensional Synergistic Optimization via Experimental Strategies
The review systematically summarizes four major experimental strategies for optimizing thermoelectric performance in binary indium chalcogenides: defect engineering, crystal orientation engineering, nano-structuring and grain size engineering, achieving breakthrough improvements in electrical and thermal transport properties through multiscale synergy. Point defects (vacancies/dopants) and dislocations effectively scatter phonons to reduce lattice thermal conductivity. For electrical transport, doping concentration can be adjusted to control carrier concentration, while doping-induced energy filtering effects enable selective carrier transport optimization. The study also highlights how grain size engineering can optimize the temperature dependence of electrical conductivity, and crystal orientation control leverages intrinsic anisotropy to fine-tune electrical and thermal transport, enhancing overall thermoelectric performance. These strategies provide critical technical pathways for advancing binary indium chalcogenides from fundamental research to device applications.

Summary and Outlook:
Despite significant progress in optimizing thermoelectric performance, practical applications of binary indium chalcogenides face three major challenges: material stability limited by temperature-induced phase transitions (e.g., In₂Te₃/In₂Se₃), difficulties in microstructure control during large-scale preparation, and cost pressures due to the scarcity of high-purity indium. Future research should focus on developing phase transition co-regulation strategies to broaden operational temperature ranges, adopting mass production techniques from systems like Bi-Sb-Te, and exploring low-cost element substitutions (e.g., Cu/Ag). Furthermore, integrating first-principles calculations, high-throughput screening, and automated characterization to establish quantitative “structure-property” models will accelerate the transition from theoretical design to device applications. Importantly, this review consolidates comprehensive computational studies, including electronic structure calculations, orbital and bonding analyses, and phonon dispersion evaluations, to provide a robust theoretical framework for understanding complex “structure-property” relationships and to guide innovative strategies for developing high-efficiency thermoelectric materials.

In summary, this review systematically summarizes the crystal structure characteristics, electronic/phonon band properties, and thermoelectric optimization strategies for binary indium chalcogenides (In-X, X = Te, Se, S), highlighting the critical role of unconventional chemical bonds (e.g., In-In bonds) in achieving ultralow thermal conductivity and high thermoelectric performance. It offers theoretical guidance and experimental paradigms for designing next-generation thermoelectric materials.

The complete study is accessible via DOI: 10.34133/research.0727