Borophosphates—A Neglected Class of Compounds: Crystal Structures of M^II[BPO₅] (M^II = Ca, Sr) and Ba₃[BP₃O₁₂]

Vertex-sharing BO4 and PO4 tetrahedra form the framework of the title compounds. Ca[BPO5] and Sr[BPO5] crystallize in the stillwellite structural type and contain tetrahedral chain anions math image[BPOmath image], which can be described as loop-branched “dreier” single chains. In Ba3[BP3O12] the math imagetetrahedral anions form “zweifach” open “vierer” single chains. Given these results and the known linking modes of tetrahedral units, it is expected that borophosphates with layered and framework structures, as well as microporous structures, can be found