Phase relations, microstructure, and valence transition studies on CaZr1–xCexTi2O7 (0.0 ≤ x ≤ 1.0) system

In this study, Ce-doped zirconolite was synthesized through high-temperature solid-state reaction at 1250 °C in air for 96 h. The crystal phase, microstructure and valence transition were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Phase relations of CaZr 1– x Ce x Ti 2 O 7 systems were determined by XRD analyses and Rietveld refinements . Four different phases are identified, namely zirconolite, perovskite , pyrochlore, and cerianite. The phase transformation (2M-zirconolite → 4M-zirconolite → Ce-pyrochlore) is caused by cations rearrangement as cerium content increases. The solubility limit of cerium ions in CaZr 1– x Ce x Ti 2 O 7 system is estimated to be approximately 0.80. Under sintering air atmosphere, partial reduction of Ce 4+ in Ce 3+ is detected in Ce 3d XPS spectra, and the ratio of Ce 3+ and Ce 4+ significantly decreases as cerium content increases.