Cooperative up-conversion processes in SrAl4O7:Yb and SrAl4O7:Yb,Tb and their dependence on charge compensation by Na

A detailed analysis of the luminescence behaviour of Yb3+-doped and Yb3+-Tb3+ co-doped strontium aluminates powders: Sr1-xYbxAl4O7 (x=0.002-0.07) and Sr1-x-yYbxTbyAl4O7 (x=0.03; y=0.002-0.02) were performed. The studies of singly doped samples show that direct excitation of Yb3+ by means of 2F7/2-2F5/2 absorption at 900-980 nm leads to Stokes Yb3+ emission in the range of 970-1130 nm as well as bluish-green Yb3+ cooperative luminescence (CL) whose energy doubles that of the NIR one. The effect of activator concentration and charge compensation through Na+ co-doping on both Yb3+ emissions were also studied. It was found that Na+ addition enhanced Stokes Yb3+ photoluminescence brightness, while the cooperative emission intensity appeared to be lower. In doubly Yb3+-Tb3+ doped materials excitation at 980 nm led to cooperative sensitization of the Tb3+ 5D4 level giving rise to its green 5D4→7FJ (J=7F6-7F3) up-conversion luminescence with the dominant component around 542 nm. The cooperative energy transfer (CET) mechanism was proposed basing on the results obtained from emission and absorption spectra, decay kinetics as well as the dependence of UC luminescence intensity on NIR excitation power.