Crystal structure and optical lattice vibrations of SbSBr crystals

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Abstract
The study of infrared reflection spectra of SbSBr crystals necessitated redetermination of its structure and revision of its description, along the lines of the isostructural compounds BiSCl, BiSBr and SbSeI. The cell constants are:a= 8.2424(6),b= 9.7556(4),c= 3.9680(2) Å,Z= 4; space groupPnam. The Sb atom is seven-coordinated by four Br and three S atoms in the form of a composite polyhedron, linked to a symmetry equivalent one. This characteristic double polyhedron is repeated along thecaxis, thus forming an infinite double chain, which is further linked to each of four similar neighboring chains.Analysis of the infrared spectra for the polarizationE⊥c, taking into account the composite polyhedra and the chains, showed for the first time in a member of the group of compounds AVBVICVII, where A = Sb, Bi; B = S, Se and C = Cl, Br, I, the presence of 10 lattice vibration bands along directions parallel to the plane (001) with symmetry B2u+ B3u, exactly as predicted by group theory for the structural model in the space groupPnam, with four formula units in the cell. This indicates the existence of relatively strong forces between the double chains in the structure.