Determination of RE2Ce2O7 pyrochlore phases from monazite–allanite ores

By using a specific chemical process, it was possible to extract a pyrochlore RE2Ce2O7 phase directly from waste materials composed of allanite–monazite minerals mixed with aluminosilicate and other phosphate phases. The pyrochlore phase is well known for its interesting thermal properties. Monazite (Ce, La, Nd, Pr, Th) PO4 is a mineral rich in rare earth elements (cerium, lanthanum). Polycrystalline waste samples were submitted to a series of chemical treatments followed by alkaline fusion. After reaction with nitric acid and addition of oxalic acid, soluble ions were separated from insoluble ones, resulting in rare earth oxalate intermediate phases. After filtration, these oxalate phases were dried and heated at 900 °C. The resulting solid was analyzed by transmission electron microscopy and X-ray diffraction. These analyses showed that the as-extracted phase was composed of a complex RE23+Ce24+O72− system, rich in cerium and lanthanum along with other rare earth elements (RE). The mean crystal structure of these complex phases was determined from Rietveld diffraction profile refinement; it was close to pyrochlore structure La2Ce2O7 with space group Fd3m. The refined cell parameter a, and the lattice distortion ɛ, calculated from profile broadening analyses, were related to the presence of RE3+ ions occupying the La site.