Synergistic effects of Ln and Fe Co-Doping on phase evolution of Ca1-xLnxZrTi2-xFexO7 (Ln = La, Nd, Gd, Ho, Yb) ceramics

Zirconolite (CaZrTi2O7) is one of the host ceramic waste forms for actinides immobilization. In this study, lanthanides were used as the surrogates for actinides and a series of solid solutions (Ca1-xLnxZrTi2-xFexO7; Ln = La, Nd, Gd, Ho, Yb) have been successfully fabricated to investigate the synergistic effects of Ln and Fe co-doping on phase evolution and solubility of Ln in zirconolite. Results show that zirconolite was found to co-exist with perovskite in the range 0.1 ≤ x ≤ 0.7 while zirconia, pseudobrookite, and perovskite become the major crystalline phases at 0.8 ≤ x ≤ 1.0 in the Ca1-xLaxZrTi2-xFexO7 ceramics. Both of zirconolite-2M and Zirconolite-3O were found in the Ca1-xNdxZrTi2-xFexO7 and Ca1-xGdxZrTi2-xFexO7 solid solutions. The phase transformation of zirconolite-2M to 3O occurs at x = 0.6 for Ca1-xNdxZrTi2-xFexO7 when this phenomenon happens at x = 0.9 for Ca1-xGdxZrTi2-xFexO7. The solubility of Ca1-xHoxZrTi2-xFexO7 to form single zirconolite-2M structure can be up to 0.9. Single zirconolite-2M was only detected within 0.1 ≤ x ≤ 0.6 in the Ca1-xYbxZrTi2-xFexO7 solid solutions. Lattice parameters of zirconolite-2M and zirconolite-3O were determined by Pawley refinement method. The lattice parameters of both zirconolite 2M and 3O gradually increase with increasing Nd and Fe contents in the Ca1-xNdxZrTi2-xFexO7 ceramics. For Ca1-xGdxZrTi2-xFexO7, the lattice parameter a has an increase with x increasing while b and c change slightly. For the Ho-Fe co-doped zirconolite-2M, the lattice parameter a shows an increasing trend, b and c demonstrate reverse trend when increasing Ho and Fe concentrations. Both b and c decrease with x increasing from 0.1 to 0.6 in the Ca1-xYbxZrTi2-xFexO7 while a has a slight change. The variation trends in the lattice parameters of zirconolite-2M phase in Ca1-xREExZrTi2-xFexO7 (REE = Nd, Gd, Ho, Yb) solid solutions are different, and such results may suggest different substitution mechanisms of lanthanide ions and Fe3+ in zirconolite.