A Series of Chalcogenide Borates RE6Ta2MgQB8O26 (RE = Sm, Eu, Gd; Q = S, Se) Featuring a B4O10 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 RE6Ta2MgQB8O26 (RE = Sm, Eu, Gd; Q = S, Se), belonging to the rare chalcogenide borates. Their structures feature an unprecedented [B4O10]8- unit comprised of one BO4 and three BO3 units. Their 3D structures are constructed by the connection between QRE6 octahedra and the ∞{[Mg(TaB4O13)2]16-} polyanionic layer. Density functional theory calculations on the electronic structure and a birefringence of 4 are also performed.