A systematic investigation of the phase assemblage and microstructure of the zirconolite CaZr1-xCexTi2O7 system

Partitioning of Ce, as a simulant for Pu, in a multiphase ceramic nuclear waste form

“…Ce/Ti Hf/Ti Y/Ti "pink" area (see Figure 6A) that zirconolite-2M transforms to zirconolite-4M then to pyrochlore with increasing Ce content. It has been reported recently that the formation of Cebearing perovskite phases is promoted by reducing conditions in multiphase zirconolite (2M and/or 4M) and perovskite ceramics, 40,41 which is consistent with our results that only trivalent Ce is incorporated in perovskite. It was further demonstrated that the amount of perovskite phase can be reduced by a subsequent heat-treatment in air via the oxidation of Ce 3+ to Ce 4+ .…”

“…Although Ce is more stable at oxidation state of 4+ in air, it is well‐known that Ce 4+ has the ability to reduce to Ce 3+ at high temperatures 36,37 . The presence of mixed Ce 3+ and Ce 4+ was observed in several multiphase ceramics sintered in air, containing pyrochlore, zirconolite, and perovskite phases, using either X‐ray photoelectron spectroscopy (XPS) 38,39 or XANES 40,41 method (though the Ce valence state in each individual mineral phase could not be distinguished based on these techniques). This study, however, is able to demonstrate that the pyrochlore phase contains exclusively Ce 4+ , whereas Ce 3+ is responsible for the stabilization of the perovskite phase.…”

“…XANES measurements revealed the presence of only trivalent Ce in the sample sintered in Ar, but both tetravalent and trivalent Ce in the sample sintered in air. Mixed Ce 3+ and Ce 4+ have been observed in quite a few Ce‐containing multiphase ceramic waste forms 9,36‐41 . However, in the absence of spatially resolved valence information, the Ce oxidation state in each individual mineral phase in those previous studies could only be deduced from crystal‐chemical principles and studies of single‐phase samples.…”

“…7 Although Ce is more stable at oxidation state of 4+ in air, it is well-known that Ce 4+ has the ability to reduce to Ce 3+ at high temperatures. 36,37 The presence of mixed Ce 3+ and Ce 4+ was observed in several multiphase ceramics sintered in air, containing pyrochlore, zirconolite, and perovskite phases, using either X-ray photoelectron spectroscopy (XPS) 38,39 or XANES 40,41 method (though the Ce valence state in T A B L E 2 Average Ca/Ti, Ce/Ti, Hf/Ti and Y/Ti concentration ratios of the "pink" and "green" areas in Figure 6A,B, respectively, extracted from SXFM elemental maps (Figure 3)…”

“…Mixed Ce 3+ and Ce 4+ have been observed in quite a few Ce-containing multiphase ceramic waste forms. 9,[36][37][38][39][40][41] However, in the absence of spatially resolved valence information, the Ce oxidation state in each individual mineral phase in those previous studies could only be deduced from crystal-chemical principles and studies of single-phase samples. In order to overcome this limitation, we have applied the fluorescence XANES imaging technique to the pyrochlore-rich multiphase ceramic sample sintered in air, and demonstrated that the major pyrochlore phase contains Ce 4+ ions exclusively (Ca 0.79 Ce 4+ 0.72 Hf 0.32 Y 0.17 Ti 1.94 O 7 ), whereas the minor perovskite phase incorporates only Ce 3+ ions (Ca 0.58 Ce 3+ 0.27 Hf 0.01 Ti 1.00 O 3 ).…”