The crystal chemistry of plutonium(iv) borophosphate

Abstract

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 BO₄⁵⁻ and tetrahedral PO₄³⁻ 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.