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, which crystallize in the orthorhombic space group Pbam (No. 55), were synthesized by a solid-state reaction. Their structures consist of puckered two-dimensional ²_∞[Pb₄Sb₄Q₉Cl]⁺ 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 confirms 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 upon 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.