Novel Fundamental Building Blocks and Site Dependent Isomorphism in the First Actinide Borophosphates

Three novel uranyl borophosphates, Ag2(NH4)3[(UO2)2{B3O­(PO4)4(PO4H)2}]­H2O (AgNBPU-1), Ag(2‑x)(NH4)3[(UO2)2­{B2P5O(20‑x)(OH)x}] (x = 1.26) (AgNBPU-2), and Ag(2‑x)(NH4)3[(UO2)2{B2P(5‑y)­AsyO(20‑x)(OH)x}] (x = 1.43, y = 2.24) (AgNBPU-3), have been prepared by the H3BO3–NH4H2PO4/NH4H2AsO4 flux method. The structure of AgNBPU-1 has an unprecedented fundamental building block (FBB), composed of three BO4 and six PO4 tetrahedra which can be written as 9□:[Φ] □⟨3□⟩□|□⟨3□⟩□|□⟨3□⟩□|. Two Ag atoms are linearly coordinated; the coordination of a third one is T-shaped. AgNBPU-2 and AgNBPU-3 are isostructural and possess a FBB of two BO4 and five TO4 (T = P, As) tetrahedra (7□:□⟨4□⟩□|□). AgNBPU-3 is a solid solution with some PO4 tetrahedra of the AgNBPU-2 end-member being substituted by AsO4. Only two out of the three independent P positions are partially occupied by As, resulting in site dependent isomorphism. The three compounds represent the first actinide borophosphates.