Abstract
The quaternary chalcogenide halides Ba3GaSe4Br and Ba3InSe4Br were prepared from reactions of binary selenides and bromides at 1073 K. Single-crystal X-ray diffraction analysis revealed that Ba3GaSe4Br adopts the Ba3GaS4Cl-type structure (orthorhombic, space group Pnma, Z = 4, a = 12.8248(6) Å, b = 9.9608(5) Å, c = 8.7690(4) Å) and Ba3InSe4Br adopts the K3SO4F-type structure (tetragonal, space group I4/mcm, Z = 4, a = 8.6888(16) Å, c = 14.950(3) Å). Both structures contain isolated [MSe4]5– tetrahedral units separated by Ba2+ and Br− ions. A structure map based on radius ratios is able to demarcate the three structure types encountered for various members of the series of chalcogenide halides Ba3MCh4X (M = Al, Ga, In; Ch = S, Se; X = Cl, Br, I). Large band gaps in Ba3GaSe4Br and Ba3InSe4Br originate from the separation of Ga–Se or In–Se bonding and antibonding levels, as revealed by electronic structure calculations. The measured optical band gaps are 1.7 eV for Ba3GaSe4Br and 1.6 eV for Ba3InSe4Br.
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