KCuBi4S7: An Expanded Bi2S3 Structure Type with a Narrow Band Gap and Ultralow Thermal Conductivity

The 6s2 lone pair of Bi3+ offers polyhedral flexibility that can be utilized in deriving new structure types with unique properties. Guided by this idea, we explored a K, Cu, and Bi/S-rich composition targeting a narrow band gap energy (Eg) and structural anisotropy. Through this strategy, we synthesized the new structure type of KCuBi4S7, which resembles the structure types of Bi2S3 and NaIn3S5, allowing us to generate the general formula Bi2S3(ABiS2)m(CuBiS2)n (A = Na, K). We observe that m and n precisely correspond to insertions of structural block fragments from KBiS2 (m → ∞) and CuBiS2 (n → ∞) into a Bi2S3-type network. Hence, we track the local dimensional expansion across this structural series of covalent networks that are globally three-dimensional (3D). Moreover, we demonstrate that incorporating Cu into a Bi2S3-type network raises the energy of the valence band maximum relative to Bi2S3, without much effect on the energy of the conduction band minimum, narrowing the Eg of Bi2S3 from 1.27 to 0.83 eV in KCuBi4S7. In addition, we link the ultralow total thermal conductivity of KCuBi4S7 (0.57-0.45 W m-1 K-1 from 300 to 730 K) to a hierarchy of crystal structure anisotropy: ionic/covalent interactions, antibonding interactions near the Fermi level, various coordination environments, and polyhedral distortions.