Incorporation of uranium in pyrochlore oxides and pressure-induced phase transitions

• We found that U ions mainly occupy the smaller cation site in U-bearing pyrochlore.
• Pyrochlore structure is not stable at pressure of more than 20 GPa.
• The quenched sample has a pyrochlore or a disordered fluorite structure.

Uranium-doped gadolinium zirconates with pyrochlore structure were studied at ambient and high-pressure conditions up to 40 GPa. The bonding environment of uranium in the structure was determined by x-ray photoelectron and Raman spectroscopies and x-ray diffraction. The uranium valence for samples prepared in air is mainly U6+, but U4+ is present in pyrochlores fabricated in an argon atmosphere. Rietveld refinement of the XRD pattern suggests that uranium ions in pyrochlores are on the 16d site in 6-fold coordination with oxygen. At pressures greater than 22 GPa, the pyrochlore structure transformed to a cotunnite-type phase. The cotunnite high-pressure phase transformed to a defect fluorite structure on the release of pressure.

In U-bearing pyrochlore, U ions mainly occupy the 16d site and replace the smaller Zr4+, part of the oxygen will occupy the 8b site, which is empty to most pyrochlores. At pressure of 22 GPa, the pyrochlore lattice is not stable and transforms to a cotunnite-type structure. The high-pressure structure is not stable and transform to a fluorite or back to the pyrochlore structure when pressure is released.Figure optionsDownload as PowerPoint slide