Unraveling U, Am&Eu ion's distribution in Ca(PO)Ffor radioactive waste immobilization and the associated U→ Euenergy transfer dynamics for tunable emission characteristics.

A combined photoluminescence (PL) and theoretical study has been performed on Ca(PO)F:U and Ca(PO)F:U,Eu compounds in order to explore Ca(PO)F as potential host for radioactive waste immobilization by understanding the distribution U, Eu and Am ions among the lattice sites and the related radiation stability. DFT based calculations on various structures with different distribution of U, Eu and Am ions showed that Eu and Am ions prefer to occupy the Ca2 sites while the highly charged U ions prefer Ca1 site. This is also supported by the PL lifetime study, which provided two lifetime components with different contribution for both U and Eu ions present at two different lattice sites. The PL study of U doped compounds confirmed the existence of U in the UO form, which makes it as a pure green emitter. Upon co-doping Eu ion, the compounds were transformed to red emitter. Further, there is an energy transfer process from Uto Eu, which shifted the CIE color coordinates towards pure red region while increasing doping level of U. This proves U as a good sensitizer for Eu ion. PL study on gamma irradiated U doped Ca(PO)F compound also showed excellent radiation stability at Ca2 site.