Effect of charge compensation on up-conversion and UV excited luminescence of Eu3+ in Yb3+-Eu3+ doped calcium aluminate CaAl4O7

Calcium aluminate powders co-doped with Yb3+ and Eu3+: Ca1−x−yYbxEuyAl4O7 (x = 0.02-0.06; y = 0.02), Ca1−x−yYbxEuyAl4O7 (x = 0.05, y = 0.01-0.07) were synthesized by a modified Pechini citrate process and their optical properties at 298 K and 77 K were investigated. Direct excitation of Yb3+ by means of 2F7/2 → 2F5/2 absorption at 980 nm leads to bluish-green luminescence centered around 488 nm due to the cooperative emission of Yb3+ and red emission of Eu3+ corresponding to 5D0 → 7FJ (J = 0-4) transitions. The near-infrared to red up-conversion luminescence of Eu3+ was assigned to a cooperative sensitization mechanism. The effect of activators concentration and charge compensation by co-doping with Na+ ions on luminescence properties were studied in detail. Na+ addition led to narrowing of luminescence bands, suggesting smaller disturbance/distribution of activator ions local symmetries. This was associated with significantly enhanced luminescence brightness of Eu3+ under UV excitation and, surprisingly, with noticeably reduced efficiency of Eu UC emission upon near-infrared excitation. It was also concluded that Yb3+ pairs transfer preferentially their excitation energy to Eu3+ ions with the local symmetry perturbed by certain defects present in the crystal lattice.