Examining Order–Disorder Structural Transition of Gd₂Zr_2–<i>x</i>Ce_<i>x</i>O₇ Using Synchrotron Techniques

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 <i>r</i>_Gd³⁺/<i>r</i>_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-<i>x</i>Ce_<i>x</i>O₇ (0 ≤ <i>x</i> ≤ 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-<i>x</i>Ce_<i>x</i>O₇. Based on synchrotron XRD data, a combination of defect fluorite and pyrochlore phases was observed for <i>x</i> = 0-0.25, the defect fluorite structure was adopted when <i>x</i> = 0.5-1.75, and the bixbyite structure was observed when <i>x</i> = 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.