Zirconate pyrochlore oxides

Zirconate pyrochlores have a low isomorphic capacity with respect tetravalent actinides. However the amount of isomorphic substitution can be increased by coupled substitution of Ca, according to the scheme 2REE = An" -i- Ca. Production of single phase ceramic can be attained only for actinide waste streams with rather simple chemical compositions. In the case of a complex waste stream composition, extra phases occur [129]. Finally, zirconate matrices require high temperatures for their synthesis. In order to prepare REE-zirconates from oxide mixtures, sintering at 1500-1600 for about 50 hours is required... 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Of these, bastnasite is the only mineral worked primarily for rare earths and both monazite and xenotime are mostly by-products of mining ilmenite, rutile, cassiterite, zircon or gold. Apatite and some multi oxide minerals like pyrochlore, euxenite, brannerite and loparite (a niobium titanate) are also commercial sources of rare earths, but production of RE from these is limited. 

In addition to fluorite structure electrolytes such as stabilised zirconia and ceria, there are many non-fluorite structure oxides which are potentially attractive for SOFC electrolyte application. These include perovskites like lanthanum gallate and to a lesser degree calcium titanate. Alternative oxides are the pyrochlores such as yttrium zirconate (YZr207)and gadolinium titanate (Gd2Ti207) [49,50], but these are only suitable in very limited oxygen pressure ranges. Therefore, the main discussion here focuses on the perovskites. 

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