Crystal Chemistry and Stabilization in Air of Brannerite, UTi₂O₆

Abstract

Brannerite, UTi 2 O 6 , can be formed only under low oxygen pressures by dry ceramic processing techniques, but the substitution of ϳ0.2 and 0.3 formula units (fu) of Ca or Gd, respectively, for U allows the stabilization of the phase in air. The Ca/Gd in brannerite provides charge compensation for some U to exist in valence states >؉4, as found by X-ray absorption spectroscopy of the U L III -edge. The maximum solubilities of Ca and trivalent rare earths in the air-fired samples, 0.3 and 0.5 fu, respectively, correspond to U having an average valence of ؉5. Ca and Gd had maximum solubilities of 0.2 and 0.45 fu, respectively, in argon-fired samples. An absorption band at 1448 nm in both air-and Ar-fired U-brannerite doped with Ca and Gd was observed using diffuse-reflectance spectroscopy and attributed to an electronic transition of U 5؉ . A similar band was observed in an annealed natural brannerite, which contained Ca, rare earths, and Th, although the band was present at ϳ1520 nm in the unannealed, X-ray amorphous sample. In synthetic ThTi 2 O 6 (thorutite, having the brannerite structure), the solubility of Ca was undetectable and that of rare earths <0.1 fu. Other ionic substitutions in synthetic brannerites involved Hf, Pu, La, and Y for U, (Gd ؉ Nb) for U ؉ Ti, and Fe in the Ti site.

Diffuse reflectance and X-ray photoelectron spectroscopy of uranium in ZrO2 and Y2Ti2O7

“…of U (Ca 0.2 Y 1.7 U 0.1 Ti 2 O 7 and Ca 0.6 Y 1.1 U 0.3 Ti 2 O 7 ) was targeted as U 5+ showed broad, probably phonon-assisted bands in the vicinity of 5000-6000 cm À1 and 7000-12,000 cm À1 , with these wavenumber ranges as expected for U 5+ [23,26]. In addition there were sharp zero-phonon lines at $6700 cm À1 (see Fig.…”

“…When Pu-doped LaPO 4 monazite ceramics are fired in air at 1000-1200°C, the substitution of divalent Ca or Pb in the trivalent La site allows the valence of Pu to change from trivalent to tetravalent [22]. For U in brannerite (UTi 2 O 6 ) in which U is normally tetravalent, the substitution of divalent or trivalent ions in the U site leads to the formation of U 5+ in both air and neutral (Ar) atmospheres [23][24][25]. Similar behaviour is found for U in zirconolite (CaZrTi 2 O 7 ) [26], and zircon [27].…”

The Synthesis and Crystal Structure of Doped Uranium Brannerite Phases U1−xMxTi2O6 (M=Ca2+, La3+, and Gd3+)

“…Diffuse reflectance spectroscopy revealed the presence of U 5+ ions, which suggested that stabilization of these doped phases was via a charge compensation mechanism rather than the formation of oxygen vacancies (7). No evidence of green fluorescence characteristic of U 6+ was observed.…”

Crystal structure and non-stoichiometry of cerium brannerite: Ce0.975Ti2O5.95

“…Natural specimens of UTi 2 O 6 exhibit chemical flexibility, with significant substitution of Pb, Ca, Th, Ln, on the U site, and Si, Al, Fe on the Ti site, commonly observed (Ln¼lanthanide) [7][8][9][10][11][12][13][14][15]. Closely related synthetic solid solutions, incorporating pentavalent uranium, have also been described by several authors [16][17][18][19]. Natural brannerites are generally found to be X-ray amorphous due to accumulated damage from the recoil daughters of U and Th a-decay [14].…”