Infrared and cooperative luminescence in Yb3+ doped calcium aluminate CaAl4O7

Two series of calcium aluminates: Singly doped CaAl4O7:Yb(x%) and co-doped CaAl4O7:Yb(x%),Na(x%) (x=0.002, 0.01, 0.02, 0.04, 0.05, 0.06, 0.07, 0.1) 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 940–980 nm leads to a reversed 2F5/2→2F7/2 transition giving Yb3+ emission in the range of 970–1130 nm as well as bluish-green up-converted (UC) luminescence whose energy doubles that of the near-IR (NIR) one. The UC luminescence was characterized by absorption and emission spectra, decay time measurements as well as its intensity dependence on the pumping power. The effect of dopant concentration and charge compensation by co-doping with Na+ ions on morphology and luminescent properties were also studied. The co-doping affected the luminescence behavior but had no influence on the powders morphology. Na+ addition led to narrowing of NIR luminescence bands, suggesting smaller disturbance of Yb3+ ions local symmetries and also significantly enhanced NIR and UC luminescence of Yb3+.