Quaternary chalcogenide halides Ba3GaSe4Br and Ba3InSe4Br

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.