A New Series of Chalcohalide Semiconductors with Composite CdBr₂/Sb₂Se₃ Lattices: Synthesis and Characterization of CdSb₂Se₃Br₂ and Indium Derivatives InSb₂S₄X (X = Cl and Br) and InM₂Se₄Br (M = Sb and Bi)

A new series of mixed-framework compounds in the chalcohalide family has been synthesized using the conventional solid-state reactions. These include CdSb 2 Se 3 Br 2 1 and its isoelectronic indium derivatives InSb 2 S 4 X (X = Cl 2 and Br 3 ) and InM 2 Se 4 Br (M = Sb 4 and Bi 5 ). Single crystals were grown from direct interaction of stoichiometric mixtures of respective elemental and binary compounds. The parent compound cadmium antimony selenobromide, CdSb 2 Se 3 Br 2, was synthesized by heating the stoichiometric mixture of CdBr 2 and Sb and Se at 450 °C followed by slow cooling. Single-crystal structure analysis was carried out by employing X-ray diffraction methods, showing that this compound crystallizes in the monoclinic crystal system ( C 2/ m, No. 12) with a = 20.998(4) Å, b = 4.0260(8) Å, c = 12.149(2) Å, β = 119.00(3)°, and V = 898.3(3) Å 3 . The framework exhibits mixed CdBr 2 and Sb 2 Se 3 sublattices, respectively, adopting the (110) and (100) NaCl-type structures. The former consists of dual octahedral chains of Cd-centered [CdBr 5 Se] units sharing cis Br edges, and the chains propagate along the b axis via sharing trans Br edges of the octahedra. The Sb 2 Se 3 sublattice is made of three edge-shared [SbSe 5 ] square-pyramidal units, features commonly seen in antimony and Sb-containing selenides. The apex selenium atoms of the aforementioned [CdBr 5 Se] unit are corner-shared with [SbSe 5 ] sublattices to form a composite slab along the (−101) plane. The lone-pair electrons of Sb 3+ cations point into the space between the stacked slabs. The indium derivatives 2 – 5 were synthesized by substituting the trivalent In 3+ for divalent Cd 2+ cations, along with a stoichiometric amount of charge-compensating halide anions. These four new In derivatives are isostructural with 1 and can be formulated as (InSX)(Sb 2 S 3 ) (X = Cl and Br), and (InSeBr)(M 2 Se 3 ) (M = Sb and Bi) with respect to the composite lattices of CdBr 2 and Sb 2 Se 3 . The UV–vis reflectance spectroscopy and band structure calculations confirm that these compounds are semiconductors with steep band gap absorption edges around 1.1–1.8 eV. In light of the observed blue shift in the absorption edge with respect to the dimensionally reduced post-transition-metal chalcogenide M 2 Q 3 sublattices, we claim quantum confinement effect of the optical band gap, which is the first example among inorganic mixed-anion compounds reported thus far.