Ba₃GeS₅ and Ba₃InS₄Cl: Interesting size effects originated from the tetrahedral anions

Two new barium chalcogenides, Ba₃GeS₅ and Ba₃InS₄Cl, were synthesized by using high temperature solid-state reactions and their structures were determined by single-crystal X-ray diffraction technique. Despite the similar chemical formula, the structures of Ba₃GeS₅ and Ba₃InS₄Cl are subtly different due to the size effects originated from the tetrahedral anions. Ba₃GeS₅ crystallizes in the orthorhombic space group Pnma (no. 62) with cell parameters of a=12.0528(9) Å, b=9.5497(7) Å and c=8.5979(6) Å, while Ba₃InS₄Cl adopts a different tetragonal system (space group: I4/mcm, no. 140, a=b=8.3613(6) Å, c=14.3806(18) Å). The measured optical band gap of Ba₃GeS₅ is 3.0 eV, a little smaller than the value of 3.42 eV in Ba₃InS₄Cl. Theoretical calculations by Wien2k are provided as well in order to better understand these results. - Graphical abstract: The polyhedral structure view for Ba₃GeS₅ and Ba₃InS₄Cl in which Ba, S and Cl atoms are plotted in purple, red and green spheres. - Highlights: • Two new barium chalcogenides, Ba₃GeS₅ and Ba₃InS₄Cl, were synthesized from the BaCl₂-flux reactions. • Their crystal structures feature discrete [MS₄] tetrahedra which embody interesting size effects. • Both compounds exhibit a band gap around 3.0 eV. • They are thermally stable up to 1073 K