Influence of synthesis atmosphere on the solid solubility of uranium at B-site of Nd2Zr2O7 pyrochlore

Abstract

In this study, the solid solubility of uranium in Nd2Zr2O7 pyrochlore and the ensuing phase evolution have been explored for nuclear waste immobilization and inert matrix fuel applications. To investigate the phase evolution upon U-substitution at the B-site of Nd2Zr2O7 pyrochlore, Nd2Zr2-xUxO7 (0 ≤ x ≤ 2) samples have been synthesized by a solid-state route under reducing and oxidizing conditions and thoroughly characterized by X-ray diffraction and Raman spectroscopy. XRD studies reveal that uranium forms a solid solution throughout the composition range adopting the pyrochlore structure at x ≤ 0.8 and the fluorite-type structure at x ≥ 1.2 under reducing conditions at 1873 K. The pyrochlore-type phase-field shrinks (x ≤ 0.4) with the concomitant extension of the fluorite-type phase-field (0.8 ≤ x ≤ 2) upon oxidation of these reduced samples in air at 1473 K. The samples synthesized under oxidizing conditions (air) at 1873 K exhibit transformation of the pyrochlore-type phase to a fluorite-type phase through a biphasic phase-field. The pyrochlore-type ordering diminishes for the oxidized samples compared to that of the reduced counterpart, which could be attributed to the oxidation of U4+ to smaller-sized U5+/U6+ and the concomitant incorporation of additional oxygen anions in the lattice for charge balance. Raman spectroscopy has been used to shed light on the local structures. Chemical analysis indicates an O/U ratio of 2.00(3) and 2.66(3) for the reduced and oxidized samples, respectively.