Examining Order–Disorder Structural Transition of Gd2Zr2–xCexO7 Using Synchrotron Techniques

Abstract

Depending on the radius ratio of A and B site cations, A₂B₂O₇-type oxides can adopt different crystal structures, including the pyrochlore-, defect fluorite-, or bixbyite-type structures. Gd₂Zr₂O₇ with a r_Gd³⁺/r_Zr⁴⁺ ratio of 1.46 is an example that exhibits a polymorphic transition between the pyrochlore- and defect fluorite-type structures. To delve deeper into these polymorphic transitions, Ce was introduced as a substitute in the B-site of Gd₂Zr₂O₇. Gd₂Zr_2–xCe_xO₇ (0 ≤ x ≤ 2) was synthesized using a coprecipitation method and annealed at 1400 °C. Previous investigations yielded disparate results regarding the structures adopted due to the challenge of detecting the low-intensity superstructural peaks associated with the pyrochlore and bixbyite structures using conventional lab-based X-ray diffraction (XRD) instruments. Utilizing synchrotron XRD with a high signal/noise ratio resolves these contradictions and aids in the structure analysis for Gd₂Zr_2–xCe_xO₇. Based on synchrotron XRD data, a combination of defect fluorite and pyrochlore phases was observed for x = 0–0.25, the defect fluorite structure was adopted when x = 0.5–1.75, and the bixbyite structure was observed when x = 2. Ce L₃ and Zr K-edge X-ray absorption near edge spectroscopy (XANES) spectra revealed that both cations were present in 4+ oxidation states.