Crystal Chemistry and Stabilization in Air of Brannerite, UTi2O6

Abstract

Brannerite, UTi2O6, can be formed only under low oxygen pressures by dry ceramic processing techniques, but the substitution of ∼0.2 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 LIII‐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 U5+. A similar band was observed in an annealed natural brannerite, which contained Ca, rare earths, and Th, although the band was present at ∼1520 nm in the unannealed, X‐ray amorphous sample. In synthetic ThTi2O6 (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.