Synthesis, Characterization, and Crystal Structure of Dominant Uranium(V) Brannerites in the UTi_2–xAl_xO₆ System

Abstract

The synthesis, characterization, and crystal structure of a novel (dominant) uranium(V) brannerite of composition U 1.09(6) Ti 1.29(3) Al 0.71(3) O 6 is reported, as determined from Rietveld analysis of the high-resolution neutron powder diffraction data. Examination of the UTi 2−x Al x O 6 system demonstrated the formation of brannerite-structured compounds with varying Al 3+ and U 5+ contents, from U 0.93(6) Ti 1.64(3) Al 0.36(3) O 6 to U 0.89(6) Ti 1.00(3) Al 1.00(3) O 6 . Substitution of Al 3+ for Ti 4+ , with U 5+ charge compensation, resulted in near-linear changes in the b and c unit cell parameters and the overall unit cell volume, as expected from ionic radii considerations. The presence of U 5+ as the dominant oxidation state in near-single-phase brannerite compositions was evidenced by complementary laboratory U L 3 -edge and high-energy-resolution fluorescence-detected U M 4 -edge X-ray absorption near-edge spectroscopy. No brannerite phase was found for compositions with Al 3+ /Ti 4+ > 1, which would require a U 6+ contribution for charge compensation. These data expand the crystal chemistry of uranium brannerites to the stabilization of dominant uranium(V) brannerites by the substitution of trivalent cations, such as Al 3+ , on the Ti 4+ site.

LnUO4‐based glass–ceramic composites as waste forms for the immobilization of lanthanide‐bearing uranium wastes

“…It is essential to understand the exact U valences in the designed waste forms as the U valences play an important role in controlling U releases to the environment 58 . Several spectroscopic techniques, such as X‐ray photoelectron spectroscopy, X‐ray absorption near‐edge spectra, and DRS, have been widely used to probe U valences in mixed oxides 59–64 . In general, uranium can be stabilized in 4+ (5f 2 electron configuration), 5+ (5f 1 ), or 6+ (5f 0 ) valence states in oxides depending on the processing redox conditions and the possible presence of other low valence cations for charge compensation 60 .…”

“…58 Several spectroscopic techniques, such as X-ray photoelectron spectroscopy, X-ray absorption near-edge spectra, and DRS, have been widely used to probe U valences in mixed oxides. [59][60][61][62][63][64] In general, uranium can be stabilized in 4+ (5f 2 electron configuration), 5+ (5f 1 ), or 6+ (5f 0 ) valence states in oxides depending on the processing redox conditions and the possible presence of other low valence cations for charge compensation. 60 In DRS, although the U 4+ ion gives sharp zero-phonon line and broad vibronic absorptions from the visible to infrared spectral range, the U 5+ ion is solely confined to the near infrared as it derives only from the crystal-field splitting of 2 F 5/2 -2 F 7/2 components (split by spin-orbit coupling) of the 2 F electronic state, [62][63][64] with the sharp electronic transitions observable from the splitting of 2 F 7/2 at 1538-833 nm (6500-12 000 cm −1 ) range depending on the local ion coordination environment.…”

Pyridine Diphosphonate Ligand for Stabilization of Tetravalent Uranium and Neptunium in Aqueous Medium under Aerobic Conditions

“…On the analytical forefront, the exciting but challenging chemistry of the f-block elements have benefitted immensely in the past with the advent of new and powerful spectroscopic techniques. − Nevertheless, the relatively old techniques such as UV–vis, fluorescence, cyclic voltametry etc., still serve as inevitable tools for the fundamental understanding of actinide chemistry. − …”

A multimodal X-ray spectroscopy investigation of uranium speciation in ThTi2O6 compounds with the brannerite structure

“…Recognising that ThTi 2 O 6 does not present redox flexibility when prepared under oxidising conditions, our intent was to control uranium speciation as the most significant variable in each solid solution, by judicious co-substitution of appropriate charge compensating species with known oxidation state. Charge compensating species were chosen based on their previously reported solid solubilities in the brannerite structure and/or similar titanate structures: Ca 2+ and Gd 3+ on the Th 4+ site 9 – 11 ; Al 3+ and Cr 3+ on the Ti 4+ site 19 , 23 , 24 . Note that compositions (Th 0.85 U 5+ 0.10 Ca 0.05 )Ti 2 O 6 and (Th 0.90 U 6+ 0.05 Ca 0.05 )Ti 2 O 6 are nominally identical to those previously investigated by Zhang et al .…”