Crystal chemistry and stabilization in air of brannerite, UTi2O6

Brannerite, UTi2O6, can be formed only under low oxygen pressures by dry ce ramic processing techniques, but the substitution of similar to0.2 and 0.3 formula units (fu) of Ca or Gd, respectively; for U allows the stabilizatio n 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 sp ectroscopy of the U L-III-edge, The maximum solubilities of Ca and trivalen t rare earths in the air-fired samples, 0.3 and 0.5 fu, respectively, corre spond to U having an average valence of +5. Ca and Gd had maximum solubilit ies 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 G d was observed using diffuse-reflectance spectroscopy and attributed to an electronic transition of U5+. A similar band was observed in an annealed na tural brannerite, which contained Ca, rare earths, and Th, although the han d was present at similar to 1520 nm in the unannealed, X-ray amorphous samp le, In synthetic ThTi2O6 (thorutite, having the brannerite structure), the solubility of Ca was undetectable and that of rare earths <0.1 fu, Other io nic substitutions in synthetic brannerites involved Hf, Pu, La, and Y for U , (Gd + Nb) for U + Ti, and Fe in the Ti site.