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

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