Effect of Na+ co-dopant and activator concentration on luminescent properties of CaGa4O7:Eu3+

Two series of calcium gallate phosphors: Ca1−xEuxGa4O7 and Ca1−2xEuxNaxGa4O7 (x=0, 0.002, 0.01, 0.02, 0.03, 0.05) were synthesized by a modified Pechini method and their optical properties at 298 and 77 K were investigated. In undoped CaGa4O7 upon 255 nm excitation a bluish white emission (λmax=500 nm) followed by an afterglow of the same color lasting for 10–20 s was observed. Eu3+-doping quenched the host-related luminescence and the characteristic red emission of the dopant with maximum at 613 nm appeared. Its excitation spectrum consisted of a broad band assigned to ligand to metal, O2−→Eu3+, charge transfer absorption and narrow lines arising from intraconfigurational transitions within the 4f6 states of Eu3+ ion. The effects of Eu3+ concentration and Na+ co-doping on the luminescence properties and decay kinetics were studied. Low temperature emission spectra showed that Eu3+ ions are positioned in environments of different symmetries. Their relative populations changed with the activator content. Co-doping with Na+ ions led to a remarkable reduction of the number of Eu3+ sites as well as to noticeable improvement of the luminescence brightness though it did not affect the decay time of the emission. The quantum efficiencies of singly doped CaGa4O7:Eu3+ were very low (in the range of 1–3.7%). Na+ co-doping improved this parameter leading to the highest efficiency of 11% for CaGa4O7:3%Eu3+,3%Na+.