Structural Characteristics and Recent Advances in Thermoelectric Binary Indium Chalcogenides.

Thermoelectric (TE) materials have garnered widespread research interest owing to their capability for direct heat-to-electricity conversion. Binary indium-based chalcogenides (In-X, X = Te, Se, S) stand out in inorganic materials by virtue of their relatively low thermal conductivity. For example, In₄Se_2.35 shows a low thermal conductivity of 0.74 W m^-1 K^-1 and an impressive zT value of 1.48 along the b-c plane at 705 K, as a result of structural anisotropy. Here, we review the structural features and recent research progress in the TE field for In-X materials. It begins by presenting the characteristics of crystal structure, electronic band structure, and phonon dispersion, aiming to microscopically understand the similarity/dissimilarity among these In-X compounds, notably the role of unconventional bonds (such as In-In) in modulating the band structures and lattice vibrations. Furthermore, TE optimization strategies of such materials were classified and discussed, including defect engineering, crystal orientation engineering, nanostructuring, and grain size engineering. The final section provides an overview of recent progress in optimizing TE properties of indium tellurides, indium selenides, and indium sulfides. An outlook is also presented on the major challenges and opportunities associated with these material systems for future TE applications. This Review is expected to provide critical insights into the development of new strategies to design binary indium-based chalcogenides as promising TE materials in the future.