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.

The dissolution in de-ionized water (DIW) at 90 and 150 °C of Cs and Ba from mechanically polished Cs-doped Ba hollandite samples is essentially congruent. The normalized Ba and Cs release rates were <0.001 g/m2/day after 56 days in DIW at 90 °C, and the Ba normalized release rate of a Cs-free sample was 0.01 g/m2/day after 56 days in DIW at 150 °C. Varying the pH between approximately 2.5 and 12.9 affected only the Ba dissolution rates of hollandite by half an order of magnitude. The dissolution rates of all species decrease with increasing leaching time due to the formation of partly impervious surface coatings of Al- and Ti-rich species. These surface coatings were investigated by scanning electron microscopy, and in some cases by cross-sectional transmission electron microscopy and x-ray photoelectron spectroscopy.

Ceramic wasteforms designed to be processed by melting in air has been developed to immobilise Cs-rich wastes. Detailed characterisation electron microscopy is presented on versions of these melted materials which are rich in Cr-, Ni-, Zn or Co- substituted titanate hollandites and which have PCT-B normalised Cs leachate concentrations of < 0.2 g/L. To assist in understanding the general crystal chemistry of titanate hollandites, this study also investigates the solubility limits of Cs in single-phase hollandite BaxCsy(M3+)2x+yTi8–2x-yO16 where M = Cr and BaxCsy(M2+)x+y/2Ti8-x-y/2O16 formulations where M = Zn, Co or Ni.