The crystal chemistry of plutonium(IV) borophosphate

In this work, we report the synthesis and characterization of a plutonium(IV) borophosphate, Pu(H₂O)₃[B₂(OH)(H₂O)(PO₄)₃] (1). The basic building unit of 1 has a B : P ratio of 2 : 3 with an equal number of BO₄ and PO₄ groups that assemble into 12-membered rings and take on a sheet topology due to presence of hydroxyl groups or a water molecule on one vertex of each BO₄ tetrahedron. This unique borophosphate anion topology is not observed in other members of the borophosphate family; it is the plutonium(IV) metal centers, rather than borate or phosphate groups, that link the sheets to form an extended framework. The presence of boron in 1 was confirmed using single crystal X-ray diffraction, electron microprobe analysis, and infrared spectroscopy. Peaks corresponding to the tetrahedral BO₄^5- and tetrahedral PO₄^3- anions were all identified in the fingerprint region (500-1500 cm^-1) of the infrared spectrum. Additionally, peaks in the higher wavenumber region corresponded to crystalline water and B-OH vibrations, providing further evidence for the water molecules surrounding plutonium in the structure and the protonation of the BO₄ tetrahedron, respectively. This compound represents the first Pu(IV) borophosphate structure and a novel borophosphate anion topology. Furthermore, the long time-frame required for crystallization of 1 and the suspected leaching of boron from the borosilicate vial used during synthesis indicate that 1 could serve as a model for the crystalline materials that are expected to form during the corrosion of vitrified nuclear waste.