Unraveling the site-specific energy transfer driven tunable emission characteristics of Eu 3+ & Tb 3+ co-doped Ca 10 (PO 4 ) 6 F 2 phosphors.

In this study we have explored Ca 10 (PO 4 ) 6 F 2 as host to develop a variety of phosphor materials with tunable emission and lifetime characteristics based on Eu 3+ and Tb 3+ as co-dopant ions and the energy transfer process involved with them. The energy transfer from the excited state of Tb 3+ ion to the 5 D 0 state of Eu 3+ 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 Eu 3+ and Tb 3+ ions and their site-selective distribution among the two different Ca-sites (CaO 9 and CaO 6 F) available. We have carried out DFT based theoretical calculation for both Eu 3+ and Tb 3+ ions in order to understand their distribution. It was observed that in cases of co-doped sample, Tb 3+ ions prefer to occupy the Ca2 site in the CaO 6 F network while Eu 3+ ions prefer Ca1 site in the CaO 9 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 Tb 3+ ions are fixed, the energy transfer from Tb 3+ ion at Ca2 site to Eu 3+ ion at Ca1 site is dominating (Tb 3+ @Ca2 → Eu 3+ @Ca1). However, for the 2 nd series of compounds, wherein the concentration Eu 3+ ions are fixed, the energy transfer process was found to occur from the excited Tb 3+ ion at Ca1 site to Eu 3+ ions at both Ca1 and Ca2 (Tb 3+ @Ca1 → Eu 3+ @Ca1 and Tb 3+ @Ca1 → Eu 3+ @Ca2). This is the first reports of its kind on site-specific energy transfer driven colour tunable emission characteristics in Eu 3+ and Tb 3+ co-doped Ca 10 (PO 4 ) 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.