Layered Mixed-Anion Chalcohalides Pb4Sb4Q9Cl2 (Q = S, Se): Puckered 2D Frameworks, Indirect Band Gaps, and Low Thermal Conductivity

Abstract

Two mixed-anion chalcohalides, Pb4Sb4S9Cl2 and Pb4Sb4Se9Cl2, were synthesized by solid-state reactions and crystallize in the orthorhombic space group Pbam (No. 55). Their structures consist of puckered two-dimensional 〖(_∞^2)[Pb_4 Sb_4 Q_9 Cl]〗^+ layers (Q = S, Se) constructed from NaCl-type ribbon motifs of Pb- and Sb-centered polyhedra, with Cl⁻ ions located between the layers. Single-crystal X-ray diffraction and synchrotron powder X-ray diffraction confirm the structural models and support the presence of mixed chalcogen/halogen occupancies at selected anion sites. X-ray photoelectron spectroscopy is consistent with the presence of Pb2+, Sb3+, Q2- (Q = S, Se), and Cl⁻. Diffuse-reflectance measurements indicate indirect optical band gaps of 1.55 eV for the sulfide and 1.01 eV for the selenide, in agreement with density functional theory calculations that reproduce the indirect-gap character and the band-gap narrowing from S to Se substitution. Both compounds exhibit low thermal conductivity at 300 K, with values of 1.1 W m⁻¹ K⁻¹ for Pb4Sb4S9Cl2 and 0.7 W m⁻¹ K⁻¹ for Pb4Sb4Se9Cl2, and the thermal conductivity decreases slightly with increasing temperature. This behavior is attributed to structural complexity, bonding heterogeneity associated with the mixed-anion framework, stereochemically active lone-pair cations, and partial halogen occupancy. These results clarify the structure-property relationships in Pb4Sb4Q9Cl2 and highlight mixed-anion design as a useful strategy for developing layered semiconducting chalcohalides with low thermal conductivity.