Effect of Ce4+-substitution at A and B sites of Nd2Zr2O7: A study for plutonium incorporation in pyrochlores

Abstract Zirconate pyrochlores have been envisaged as promising candidates for inert matrix fuel applications and immobilization of nuclear waste. Therefore, Ce4+-substituted Nd2Zr2O7 samples with general formula [Nd1-xCex]2Zr2O7+δ (0.0 ≤ x ≤ 0.5) and Nd2[Zr1-xCex]2O7 (0.0 ≤ x ≤ 1.0) were synthesized by solid state route and thoroughly characterized by X-ray diffraction (XRD) in order to mimic plutonium incorporation at A- and B-sites of Nd2Zr2O7 pyrochlore which are crystallographically distinct. The local structure has also been probed by Raman spectroscopy and X-ray absorption near edge structure (XANES) spectroscopy. Interestingly, the substitution of Ce4+ at different sites yields distinct phase relations. Ce4+-substitution at A- (Nd3+) site retains pyrochlore-type phase up to nominal composition with x = 0.4 and F-type phase was observed for composition with x = 0.5. However, Ce4+-substitution at B- (Zr4+) site demonstrated transformation of pyrochlore-type phase to F-type phase through a biphasic phase field consisting of these two phases. Raman spectroscopic studies on Nd2[Zr1-xCex]2O7 (0.0 ≤ x ≤ 1.0) system clearly indicated existence of C-type ordering in largely F-type phases for compositions with x = 0.6 and beyond. XANES studies exhibited a change in Zr local coordination upon successive Ce4+-substitution at ‘Nd’ site and pointed towards increase in local disorder across the pyrochlore phase regime. Ce LIII edge XANES also revealed significant changes in ‘Ce’ local environment in Ce-substituted samples.