Crystal structures and electronic structures of SrSiSe₃ and BaGeSe₃

The structural diversity of ternary chalcogenides with the ABQ₃ stoichiometry remains incompletely understood, particularly in systems where computational predictions have proposed structures that differ from experimentally accessible phases. This study reports the experimental crystal structures of the selenides SrSiSe₃ and BaGeSe₃, with the former being reported for the first time, and the latter revealing a unique structure type that differs from the previously calculated one. SrSiSe₃ adopts the centrosymmetric monoclinic space group P2₁/c with lattice parameters a = 6.6948(2) Å, b = 17.4606(4) Å, c = 15.8647(3) Å, β = 117.0100(10)°, and features pairs of edge-sharing SiSe₄ tetrahedra. BaGeSe₃ adopts the same space group, P2₁/c, with a = 15.6603(7) Å, b = 6.0379(3) Å, c = 14.0072(6) Å, and β = 116.3830(10)°, but exhibits a distinctly different structural motif consisting of chains of corner-sharing GeSe₄ tetrahedra. Comparisons with reported and calculated structures across the ABQ₃ family highlight the substantial structural diversity accessible within this simple stoichiometry and demonstrate the importance of experimental structure determinations or global structure predictions in evaluating unknown structures. DFT calculations further indicate that SrSiSe₃ and BaGeSe₃ are semiconductors with calculated band gaps of 2.15 eV and 1.65 eV, respectively, placing both materials within the visible energy range. These results expand the known structural chemistry of ABQ₃ ternary selenides and provide experimentally validated structures for assessing structure-property relationships and the reliability of computational predictions in this family.