Flux Crystal Growth of the Extended Structure Pu(V) Borate Na₂(PuO₂)(BO₃)

Abstract

As part of our exploration of plutonium-containing materials as potential nuclear waste forms, we report the first extended structure Pu(V) material and the first Pu(V) borate. Crystals of Na 2 (PuO 2 )(BO 3 ) were grown out of mixed hydroxide/boric acid flux and found to crystallize in the orthorhombic space group Cmcm with lattice parameters of a = 9.9067(4) Å, b = 6.5909(2) Å, and c = 6.9724(2) Å. Na 2 (PuO 2 )(BO 3 ) adopts a layered structure in which layers of PuO 2 (BO 3 ) 2– are separated by sodium cations. Plutonium is found in a pentagonal bipyramidal coordination environment, with axial Pu(V)–O plutonyl bond lengths of 1.876(3) Å and equatorial Pu–O bond lengths ranging from 2.325(5) to 2.467(3) Å. We find that the Pu(V)–O plutonyl bond lengths are approximately 0.1 Å longer than the reported Pu(VI)–O plutonyl bond lengths and shorter by approximately 0.033 Å than the corresponding U(V) uranyl bond lengths. Raman spectroscopy on single crystals was used to determine the PuO 2 + plutonyl stretching and the equatorial breathing mode frequencies of the pentagonal bipyramidal coordination environment around plutonium. Density functional theory calculations were used to calculate the Raman spectrum to help identify the Raman bands at 690 and 630 cm –1 as corresponding to the plutonyl(V) ν1 stretch and the equatorial PuO 5 breathing mode, respectively. UV–vis measurements on single crystals indicate semiconducting behavior with a band gap of ∼2.60 eV.