Phase evolution and valence state change in Gd-Ce co-doped zirconolite glass-ceramics

Zirconolite based glass-ceramic is widely researched for the immobilization of actinide-rich high-level waste (HLW). The research of the co-doping effect has important guiding significance for the waste disposal processes. In this study, the co-doping effects of gadolinium and cerium in the zirconolite glass-ceramics were investigated systematically by using X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The results revealed that the high amount of Gd can be well incorporated in the Ce-bearing zirconolite particles. The zirconolite was transformed from 2M to 3T structure with the increase of the Gd-doping concentration. The further addition of Gd caused the formation of Gd ₂Ti ₂O ₇ pyrochlore with the doping concentration of about 10.0 wt%, while the zirconolite was still the dominant ceramic phase. The reduction of Ce from Ce ⁴⁺ to Ce ³⁺ was firstly inhibited and then stabilized with the doping of Gd in this system, indicated that the redox equilibrium of cerium (Ce ⁴⁺ ↔ Ce ³⁺) was established in the sample with the high amount of Gd doping. This work aimed to further understand the co-doping effect of actinides in zirconolite glass-ceramics, which is expected to provide an important basic scientific basis for underground disposal of HLW.