Synthesis, Structure, and Properties of a New Family of Mixed-Framework Chalcohalide Semiconductors: CdSbS₂X (X = Cl, Br), CdBiS₂X (X = Cl, Br), and CdBiSe₂X (X = Br, I)

A new family of quaternary mixed-framework chalcohalide semiconductors has been synthesized by conventional solid-state reactions in an intermediate temperature region of 400−430 °C. These include CdSbS 2 Cl ( 1 ), CdSbS 2 Br ( 2 ), CdBiS 2 Cl ( 3 ), CdBiS 2 Br ( 4 ), CdBiSe 2 Br ( 5 ), and CdBiSe 2 I ( 6 ). Single-crystal structure analyses of this compound series reveal two types of crystal structures depending upon the combination of chalcohalide anions, and they crystallize in an orthorhombic Pnma (type I) and monoclinic C 2/ m (type II) space group. Type I is adopted by two sulfochlorides, 1 and 3, and one selenobromide, 5 . Type II is adopted by two sulfobromides, 2 and 4, and one selenoiodide, 6 . Both structure types have slabs built of corner and edge shared Cd-centered CdQ 6 - x X x (Q = S, Se; X = Cl, Br, I; x = 0, 2, 4) octahedra that resemble the (110) plane of a distorted NaCl-type structure. In the type I structure, this slab contains only CdQ 4 X 2 octahedra, while type II shows alternating CdQ 6 and CdQ 2 X 4 octahedra. In both structure types, each M 3+ (M = Sb, Bi) cation of CdMQ 2 X forms a distorted square pyramid, MQ 5, as found in M 2 Q 3 . The MQ 5 unit is weakly coordinated to three X atoms to form a distorted bicapped trigonal prism, MQ 5 X 3 . In forming the extended network structure, the CdQ 6 - x X x and MQ 5 X 3 units are solely linked through chalcogenide anions. The Fourier transform infrared spectroscopy studies suggest that these chalcohalides are transparent in the mid-IR region (1400−4000 cm - 1 ). The UV−vis spectroscopy results in a band gap ranging from 1.3 to 2.2 eV, showing a red shift with respect to the corresponding binary chalcogenides CdQ. The results of tight-binding electronic band structure calculations suggest that the origin of this red shift is due to the lone-pair effects from Sb and Bi.