A New Series of Chalcohalide Semiconductors with Composite CdBr2/Sb2Se3Lattices: Synthesis and Characterization of CdSb2Se3Br2and 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 2Se 3Br 21and its isoelectronic indium derivatives InSb 2S 4X (X = Cl 2and Br 3) and InM 2Se 4Br (M = Sb 4and 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 2Se 3Br 2, was synthesized by heating the stoichiometric mixture of CdBr 2and 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 ( C2/ 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 2and Sb 2Se 3sublattices, respectively, adopting the (110) and (100) NaCl-type structures. The former consists of dual octahedral chains of Cd-centered [CdBr 5Se] units sharing cisBr edges, and the chains propagate along the baxis via sharing transBr edges of the octahedra. The Sb 2Se 3sublattice 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 5Se] 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– 5were 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 1and can be formulated as (InSX)(Sb 2S 3) (X = Cl and Br), and (InSeBr)(M 2Se 3) (M = Sb and Bi) with respect to the composite lattices of CdBr 2and Sb 2Se 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 2Q 3sublattices, we claim quantum confinement effect of the optical band gap, which is the first example among inorganic mixed-anion compounds reported thus far. [ABSTRACT FROM AUTHOR]