A Series of Chalcogenide Borates RETaMgQBO (RE = Sm, Eu, Gd; Q = S, Se) Featuring a BO Cluster.

New compounds with multiple anions are receiving increasing interest in view of their diverse structures and physical properties. Here we report four isostructural hexanary RETaMgQBO (RE = Sm, Eu, Gd; Q = S, Se), belonging to the rare chalcogenide borates. Their structures feature an unprecedented [BO] unit comprised of one BO and three BO units. Their 3D structures are constructed by the connection between QRE octahedra and the {[Mg(TaBO)]} polyanionic layer. Density functional theory calculations on the electronic structure and a birefringence of 4 are also performed.