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

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 Na2(PuO2)(BO3) 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. Na2(PuO2)(BO3) adopts a layered structure in which layers of PuO2(BO3)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 PuO2+ 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 PuO5 breathing mode, respectively. UV-vis measurements on single crystals indicate semiconducting behavior with a band gap of ∼2.60 eV.

Crystallization of A3Ln(BO3)2 (A = Na, K; Ln = Lanthanide) from a Boric Acid Containing Hydroxide Melt: Synthesis and Investigation of Lanthanide Borates as Potential Nuclear Waste Forms.

A series of alkali metal rare-earth borates were prepared via high-temperature flux crystal growth, and their structures were characterized by single crystal X-ray diffraction (SXRD). Na3Ln(BO3)2 (Ln = La-Lu) crystallize in the monoclinic space group P21/n, the potassium series K3Ln(BO3)2 (Ln = La-Tb) crystallize in the orthorhombic space group Pnma, while the Ln = Dy, Ho, Tm, Yb analogues crystallize in the orthorhombic space group Pnnm. To demonstrate the generality of this synthetic technique, high-entropy oxide (HEO) compositions K3Nd0.15(1)Eu0.20(1)Gd0.20(1)Dy0.22(1)Ho0.23(1)(BO3)2 and K3Nd0.26(1)Eu0.29(1)Ho0.22(1)Tm0.14(1)Yb0.10(1)(BO3)2 were obtained in single crystal form. Radiation damage investigations determined that these borates have a high radiation damage tolerance. To assess whether trivalent actinide analogues of Na3Ln(BO3)2 and K3Ln(BO3)2 would be stable, density functional theory was used to calculate their enthalpies of formation, which are favorable.