Terbium(III) doped silica-xerogels: effect of aluminium(III) co-doping

Monolithic silica xerogels doped with different concentrations of Tb3+ and Al3+ (xTbO(1.5)-x6AlO(1.5)-SiO2, x = 2 x 10(-4), 4 x 10(-4), 5 x 10(-3)) were prepared by a sol-gel route: Mixtures of tetra-methoxy-silane, Tb(NO3)(3) . 5H(2)O, Al(NO3)(3) . 9H(2)O, ethanol, and water were condensed at 60 degrees C using HNO3 as catalyst. The samples were sintered at 950 degrees C for 86 h in air. The Raman spectra indicate several structural differences among the samples and show that there is a compositional dependence of the gel glass transformation temperature. The sample with x = 5 x 10(-3) is glasslike on the outer part showing onset of crystallisation in the inner part of the sample. The samples with smallest dopant concentrations (x = 2 x 10(-4), 4 x 10(-4)) contain pores. All samples luminesce from the D-5(4) and D-5(3) states of Tb3+ with excitation at 355 nm. The 5D3 luminescence increases with respect to the D-5(4) luminescence with increasing dopant concentration, indicating that the larger Al3+ concentrations are effective in dispersing Tb3+ ions. This result is also confirmed by the analysis of the decay of the D-5(3) and D-5(4) luminescence. (C) 1999 Elsevier Science B.V. All rights reserved.