Ultralow Thermal Conductivity in Layered CuGe₂Se₃

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 CuGe₂Se₃, whose synthesis, unique crystal structure, and transport properties are reported herein. The giant two-dimensional (2D) structure of CuGe₂Se₃, 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^-1K^-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.