Phase evolution, solubility and substitution behavior of CaZrTi2O7–Ho2Ti2O7 ceramics for potential nuclear waste immobilization

Zirconolite and pyrochlore have been considered as the potential immobilization matrix materials of minor actinides due to their high solubility, radiation tolerance and chemical durability. In this work, a series of Ca 1-x Ho 2x Zr 1-x Ti 2 O 7 (0.1 ≤ x ≤ 1.0) ceramics were prepared by solid state synthesis and sintering at 1400 °C for 48 h twice. Powder X-ray Diffraction (XRD), Backscattered Scanning Electron Microscopy with Energy-Dispersive X-ray spectroscopy (BSEM-EDX), Transmission Electron Microscopy (TEM) were employed to investigate the phase evolution, solubility, and substitution mechanism of Ho. The results displayed that zirconolite-2M would be transformed to zirconolite-4M and then to pyrochlore with increasing Ho concentration. The initial phase transformation of zirconolite-2M to zirconolite-4M, subsequently zirconolite-4M to pyrochlore were observed in x = 0.2 and 0.4 samples, respectively. Zirconolite-4M could be synthesised at x = 0.38 sample with trace perovskite. Moreover, the semi-quantitively solubility limits of Ho in zirconolite-2M and zirconolite-4M were determined to be 27 and 41.5 at.%, respectively. Single-phase pyrochlore were fabricated after 70 at.%. In addition, the novel substitution mechanism was revealed by Rietveld refinement with Ho preferentially replacing the Ca of zirconolite-2M so that some replaced Ca occupied vacant Zr site. For pyrochlore, Ho, Ca and Zr occupied A site (16d) and Ti entered in B site (16c). The findings would provide more insights into zirconolite–pyrochlore ceramic system.