Effects of ionic radius on phase evolution in Ln-Al co-doped Ca1-xLnxZrTi2-xAlxO7 (Ln = La, Nd, Gd, Ho, Yb) solid solutions

Zirconolite ceramic has been considered as a promising matrix to dispose high-level radioactive waste due to its excellent performance in immobilizing radionuclides. In this work, a series of zirconate solid solutions with stoichiometric Ca1-xLnxZrTi2-xAlxO7 (Ln = La, Nd, Gd, Ho, Yb; x = 0.1-1) were systematically studied to investigate the radius effect on their phase evolution. Powder X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-Ray spectrometry (SEM-EDX) were used to characterize the products. XRD and SEM results show that complete solid solutions of Ln and Al in zirconolite phase for Ca1-xLnxZrTi2-xAlxO7 cannot found. In the Ca1-xLaxZrTi2-xAlxO7 ceramics, single zirconolite phase cannot form, instead of multiple phases, such as zirconolite-2M, zirconia, perovskite and LaTi2Al9O19. In the Nd-Al co-doping ceramics, nearly single zirconolite-2M and zirconolite-3O were found at x ≤ 0.6 and 0.8 ≤ x ≤ 0.9, respectively. The miscibility gap between zirconolite-2M and 3O was found at x = 0.7. Single zirconolite-2M formed in the Gd-Al, Ho-Al and Yb-Al co-doped ceramics can only be detected in a compositional range of 0.1 ≤ x ≤ 0.8. Higher incorporation contents in these three series can form an additional phase cubic zirconia which is usually a ceramic waste form for radionuclides. Based on the XRD data, lattice parameters of zirconolite-2M and zirconolite-3O were calculated by Pawley refinement method. The evolution of lattice parameters of zirconolite-2M shows great difference between different lanthanide ions, indicating different substitution mechanisms in the Ln-Al co-doped zirconolite-2M.