Unraveling the site-specific energy transfer driven tunable emission characteristics of Eu3+ & Tb3+ co-doped Ca10(PO4)6F2 phosphors

In this study we have explored Ca-10(PO4)(6)F-2 as host to develop a variety of phosphor materials with tunable emission and lifetime characteristics based on Eu3+ and Tb3+ as co-dopant ions and the energy transfer process involved with them. The energy transfer from the excited state of Tb3+ ion to the D-5(0) state of Eu3+ makes it possible to tune the colour characteristics from yellow to orange to red. Further, such energy transfer process is highly dependent on the concentration of Eu3+ and Tb3+ ions and their site-selective distribution among the two different Ca-sites (CaO9 and CaO6F) available. We have carried out DFT based theoretical calculation for both Eu3+ and Tb3+ ions in order to understand their distribution. It was observed that in cases of co-doped sample, Tb3+ ions prefer to occupy the Ca2 site in the CaO6F network while Eu3+ ions prefer Ca1 site in the CaO9 network. This distribution has significant impact on the lifetime values and the energy transfer process as observed in the experimental photoluminescence lifetime values. We have observed that for the 1(st) series of compounds, wherein the concentration Tb3+ ions are fixed, the energy transfer from Tb3+ ion at Ca2 site to Eu3+ ion at Ca1 site is dominating (Tb3+@Ca2 -> Eu3+@Ca1). However, for the 2(nd) series of compounds, wherein the concentration Eu3+ ions are fixed, the energy transfer process was found to occur from the excited Tb3+ ion at Ca1 site to Eu3+ ions at both Ca1 and Ca2 (Tb3+@Ca1 -> Eu3+@Ca1 and Tb3+@Ca1 -> Eu3+@Ca2). This is the first reports of its kind on site-specific energy transfer driven colour tunable emission characteristics in Eu3+ and Tb3+ co-doped Ca-10(PO4)(6)F-2 phosphor and it will pave the way for the future development of effective colour tunable phosphor materials based on a single host and same co-dopant ions.

Automated summary

This study explored the development of phosphor materials with tunable emission characteristics based on Eu3+ and Tb3+ co-dopants in Ca-10(PO4)(6)F-2 host. It was found that the selective distribution of Eu3+ and Tb3+ ions at specific sites significantly impacts the emission characteristics and lifetime values.