Cooperative energy transfer in Yb3+-Tb3+ co-doped CaAl4O7 upconverting phosphor

Tb3+ and Yb3+ co-doped calcium aluminate powders: Ca1−x−yYbxTbyAl4O7 (x = 0.02-0.06; y = 0.02), Ca1−x−yYbxTbyAl4O7 (x = 0.03, y = 0.01-0.07), Ca1−x−yYbxTbyAl4O7 (x = 0.01-0.05, y = 0.01) were synthesized by a modified Pechini citrate process and their optical properties were investigated. Upon excitation at 245 nm all studied samples yielded relatively strong Tb3+ luminescence corresponding to the 5D4 → 7FJ and 5D3 → 7FJ transitions with the dominant component around 542 nm. The direct excitation of two Yb3+ ions at 980 nm led to cooperative sensitization of the Tb3+5D4 level giving rise to its green up-conversion luminescence. The up-conversion emission was also observed from higher 5D3 level of Tb3+, which is rather rare in high phonon frequency host materials. The effect of activator concentration and charge compensation by co-doping with Na+ ions on luminescence properties were studied. It was found that Na+ addition lowers disturbance/distribution of activator ions local symmetries and enhanced Stokes Tb3+ photoluminescence brightness, while the up-conversion emission intensity does not appear to change noticeably then.