Higher valency ion substitution causing different fluorite-derived structures in CaZr1-xNdxTi2-xNbxO7 (0.05 <= x <= 1) solid solution

Fluorite-derived A(2)B(2)O(7) and ABC(2)O(7) structures, i.e. pyrochlore and zirconolite, are promising ceramic matrices for high-level radioactive waste immobilization, especially for minor actinides. Though substitutions of low valent cations were reported by many studies, fewer researches were carried out on the incorporation of high valency elements on these fluorite-derived matrices. In this study, a series of CaZr1-xNdxTi2-xNbxO7 (x = 0.05-1.0) ceramics was investigated to unravel the effect of Nb5+ - one of the main impurities in natural zirconolite, on the evolutions of crystal structure, substitution mechanism and phase transformation. X-ray photoelectron spectroscopy was used to confirm the existence of Nb5+ in the samples. Fluorite-derived structures (zirconolite-2M, 3T, 4 M and pyrochlore) with increasing Nb5+ contents simultaneously appeared as single-phase or multi-phase assemblages in the matrices, revealed by the combination of synchrotron X-ray diffraction and scanning electron microscopy-energy dispersive x-ray spectroscopy. Almost single-phase zirconolite-2M and pyrochlore were observed only in the ranges of 0.05 <= x <= 0.1 and 0.5 <= x <= 0.9, respectively. Rietveld structural analysis of the zirconolite-2M phase showed that Nd3+ ions preferably occupied the Zr sites while Nb5+ ions substituted the Ti sites. This result is different from the mechanisms reported in the literature when lower valency (<=+3) was as charge compensator or there was no charge compensator in the REE doped zirconolite (REE = rare earth element). In the pyrochlore structure, the Ca2+, Nd3+ and Zr4+ ions occupied the A sublattice while Ti4+ and Nb5+ ions filled in the B sublattice, and there is charge compensation between A and B sublattices. The results in this study showed a different charge compensation mechanism of the fluorite-derived structures when higher valency elements incorporate into the lattice and will be very helpful for high-level radioactive waste immobilization.