Three new chalcohalides, Ba sub(4)Ge sub(2)PbS sub(8 )Br sub(2), Ba sub(4)Ge sub(2)PbSe sub( 8)Br sub(2) and Ba sub(4)Ge sub(2)SnS sub(8 )Br sub(2): Syntheses, crystal structures, band gaps, and electronic structures

Single crystals of three new chalcohalides: Ba sub(4)Ge sub(2)PbS sub(8 )Br sub(2), Ba sub(4)Ge sub(2)PbSe sub( 8)Br sub(2) and Ba sub(4)Ge sub(2)SnS sub(8 )Br sub(2) have been synthesized for the first time. These isostructural compounds crystallize in the orthorhombic space group Pnma. In the structure, the tetra-valent Ge atom is tetrahedrally coordinated with four Q(Q= S,Se) atoms, while the bi-valent M atom (M = Pb,Sn) is coordinated with an obviously distorted octahedron of five Q (Q = S, Se) atoms and one Br atom, showing the stereochemical activity of the ns super(2) lone pair electron. The MQsBr (M = Sn.Pb; Q=S,Se) distorted octahedra and the GeQ sub(4) (Q = S,Se) tetrahedra are connected to each other to form a three-dimensional framework with channels occupied by Ba super(2+) cations. Based on UV-vis-NIR spectroscopy measurements and the electronic structure calculations, Ba sub(4)Ge sub(2)PbS sub(8 )Br sub(2), Ba sub(4)Ge sub(2)PbSe sub( 8)Br sub(2) and Ba sub(4)Ge sub(2)SnS sub(8 )Br sub(2) have indirect band gaps of 2.054, 1.952, and 2.066 eV respectively, which are mainly determined by the orbitals from the Ge, M and Q atoms (M = Pb,Sn; Q= S,Se).