Unraveling U6+, Am3+&Eu3+ ion's distribution in Ca10(PO4)6F2for 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 Ca10(PO4)6F2:U6+ and Ca10(PO4)6F2:U6+,Eu3+ compounds in order to explore Ca10(PO4)6F2 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 co-doping 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 Ca10(PO4)6F2 compound also showed excellent radiation stability at Ca2 site.