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[en] Highlights: • Speciation of uranium in gadolinium zirconate. • Uranium stabilizes as U(IV) as well as U(VI). • U(VI) stabilizes as UO66− ion in Gd2Zr2O7. • XPS also confirms the presence of U in +6/+4 state but quantification is not possible. • Positron lifetime indicated formation of antisite defects. Based on photoluminescence spectroscopy it was inferred that uranium stabilizes as both U(IV) as well as U(VI) in Gd2Zr2O7 which was also corroborated using X-ray photo electron spectroscopy (XPS). Absence of equidistant vibronic structure in emission spectrum of Gd1.98U0.02Zr2O7 confirmed that U(VI) stabilizes in the form of UO66−. Based on luminescence lifetime it was inferred that majority of UO66− stabilizes at both Gd3+/Zr4+ whereas U4+ stabilizes only at Zr4+ sites. The positron lifetime doesn’t change on uranium doping indicating the formation of antisite defect. Infact it is this antisite defect in Gd1.98U0.02Zr2O7 which favours the stabilization of its fluorite phase.