The ZrO2–CeO2 system is fundamental to various technological applications, yet unresolved questions persist regarding cation miscibility and the occurrence of metastable phases in the Zr1–xCexO2 phase diagram. This work addresses these gaps through a comprehensive investigation of Zr1–xCexO2 compositions with varying cerium concentrations and incorporating Eu3+ as a luminescent probe. Synchrotron powder X-ray diffraction analysis unveiled a miscibility gap between 20 and 50 mol % cerium. Beyond this gap, the formation of solid solutions and multiple crystalline phases was observed, including tetragonal prime (t′) and tetragonal double prime (t″) structures, depending on cerium content. Raman investigations revealed a unique distortion band in all compositions containing the t′ phase. Our high energy resolution fluorescence detected X-ray absorption near edge structure spectroscopy (HERFD-XANES) analysis implies that this feature results from oxygen ion displacement in the t′ structure. Luminescence spectroscopy of the europium environment revealed distinct excitation and emission spectra across the various crystal phases, enabling unambiguous identification of all metastable phases. These findings highlight the complex polymorphism of the ZrO2–CeO2 system. The ability to precisely control phase composition offers significant potential for optimizing properties for diverse applications, including oxygen sensors, three-way catalysts, and solid oxide fuel cells for clean, sustainable energy generation.
