| Abstract
| - New chalcohalide compounds exhibit a composite lattice made of alternately mixed nanostructures of the CdBr2 and Sb2Se3 types. Theoretical band structure calculations suggest that the orbitals of the Sb2Se3 sublattice are responsible for the bands near Fermi. The quantum confinement effect was observed for the first time in these dimensionally reduced antimony and bismuth chalcogenides of extended solids.
- A new series of mixed-framework compounds in the chalcohalide family has been synthesized using the conventional solid-state reactions. These include CdSb2Se3Br21 and its isoelectronic indium derivatives InSb2S4X (X = Cl 2 and Br 3) and InM2Se4Br (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, CdSb2Se3Br2, was synthesized by heating the stoichiometric mixture of CdBr2 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 (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) Å3. The framework exhibits mixed CdBr2 and Sb2Se3 sublattices, respectively, adopting the (110) and (100) NaCl-type structures. The former consists of dual octahedral chains of Cd-centered [CdBr5Se] units sharing cis Br edges, and the chains propagate along the b axis via sharing trans Br edges of the octahedra. The Sb2Se3 sublattice is made of three edge-shared [SbSe5] square-pyramidal units, features commonly seen in antimony and Sb-containing selenides. The apex selenium atoms of the aforementioned [CdBr5Se] unit are corner-shared with [SbSe5] sublattices to form a composite slab along the (−101) plane. The lone-pair electrons of Sb3+ cations point into the space between the stacked slabs. The indium derivatives 2-5 were synthesized by substituting the trivalent In3+ for divalent Cd2+ 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)(Sb2S3) (X = Cl and Br), and (InSeBr)(M2Se3) (M = Sb and Bi) with respect to the composite lattices of CdBr2 and Sb2Se3. 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 M2Q3 sublattices, we claim quantum confinement effect of the optical band gap, which is the first example among inorganic mixed-anion compounds reported thus far.
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