Unraveling U6+, Am3+&Eu3+ ion's distribution in Ca10(PO4)6F2 for radioactive waste immobilization and the associated U6+ → Eu3+ energy transfer dynamics for tunable emission characteristics

A combined photoluminescence (PL) and theoretical study has been performed on Ca-10(PO4)(6)F-2:U6+ and Ca-10(PO4)(6)F-2:U6+,Eu3+ compounds in order to explore Ca-10(PO4)(6)F-2 as potential host for radioactive waste immobilization by understanding the distribution U6+, Eu3+ and Am3+ ions among the lattice sites and the related radiation stability. DFT based calculations on various structures with different distribution of U6+, Eu3+ and Am3+ ions showed that Eu3+ and Am3+ ions prefer to occupy the Ca2 sites while the highly charged U6+ ions prefer Ca1 site. This is also supported by the PL lifetime study, which provided two lifetime components with different contribution for both U6+ and Eu3+ ions present at two different lattice sites. The PL study of U6+ doped compounds confirmed the existence of U in the UO22+ form, which makes it as a pure green emitter. Upon codoping Eu3+ ion, the compounds were transformed to red emitter. Further, there is an energy transfer process from U6+ to Eu3+, which shifted the CIE color coordinates towards pure red region while increasing doping level of U6+. This proves U6+ as a good sensitizer for Eu3+ ion. PL study on gamma irradiated U6+ doped Ca-10(PO4)(6)F-2 compound also showed excellent radiation stability at Ca2 site.