Ultralow Thermal Conductivity in Layered CuGe ₂ Se ₃

Unraveling the relationship between thermodynamic factors, interatomic interactions, and electronic structure remains a crucial yet elusive challenge in the discovery of novel materials in solid‐state (SS) chemistry. In the quest for new thermoelectric (TE) materials, we overcame that fundamental problem for the case of CuGe 2 Se 3 , whose synthesis, unique crystal structure, and transport properties are reported herein. The giant two‐dimensional (2D) structure of CuGe 2 Se 3 , consisting of Se/(Cu─Ge)/Se and Se/(Ge─Ge)/Se slabs stacked along the c ‐axis, exhibits short Cu─Ge and Ge─Ge interactions, as evidenced by single‐crystal X‐ray diffraction (SCXRD) and SS NMR spectroscopic studies. These homopolar bonds might be surprising, as such interactions are rarely observed in group IV‐chalcogenides. The compound is thermally stable up to ∼823 K. Transport properties measurements revealed a high Seebeck coefficient (∼373.6 µV·K −1 ) and ultralow thermal conductivity (∼0.35 W·m −1 K −1 ) at 755 K, ascribed to its weak bonding interactions. We followed up with a theoretical analysis to gain insight into its structural peculiarities, focusing on vibrational properties and the nature of chemical bonding. The formation of Ge─Ge bonds is favored in light of the presence of multicenter bonds, which receive contributions from stereochemically non‐active Ge lone pairs.