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)­As_yO_(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.