Phase transition in Eu2Zr2–xO7–2x (x = 0–1) solid solutions: A combined structural and spectroscopic study

Zirconate pyrochlores have been extensively studied as candidate waste forms for the immobilization of actinide wastes due to their flexible crystal chemistry, excellent structural stability, and exceptional radiation tolerance. Herein, we report the synthesis of a series of compounds Eu2Zr2–xO7–2x (x = 0–1, step = 0.125) using a co‐precipitation route to form solid solutions between the two end members, Eu2Zr2O7 pyrochlore and Eu2ZrO5 defect fluorite. It is important to understand the pyrochlore to defect fluorite transformation as a function of Zr/Ln molar ratio. The structural investigation by x‐ray diffraction has shown the increasing cation disorder and corresponding structural transitions from the ordered pyrochlore to disordered pyrochlore, then to defect fluorite. While diffuse reflectance spectroscopy provides similar optical absorption features reflecting the presence of Eu(III) ion, Raman spectroscopy has revealed more details on the phase transition and short‐range structures. This work highlights the structural flexibility in zirconate materials as potential nuclear waste forms for the immobilization of actinide‐rich waste streams, especially suitable for the separated minor actinides. We report the fabrication of a series of solid solutions, Eu2Zr2–xO7–2x (x = 0–1, step = 0.125), using a co‐precipitation route. Subsequently, the phase transitions from ordered pyrochlore Eu2Zr2O7 to disordered pyrochlore Eu2Zr1.5O6, then to defect fluorite Eu2ZrO5 have been studied using both structural and spectroscopic techniques.