Enhanced phase miscibility and luminescence by inducing oxygen vacancies in Ce1−xEuxO2−δ under a strongly reducing atmosphere

► A novel synthetic process to achieve complete solid solubility in Ce1-xRExO2−δ. ► Micrometer-sized particles with enhanced photoluminescence. ► An excess of oxygen vacancies assisting in producing complete solubility. ► Phase separation with the migration of oxygen vacancies and Eu3+ to the surfaces.