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

Monolithic silica xerogels doped with different concentrations of Tb3+ and Al3+ (xTbO1.5-x6AlO1.5-SiO2, x = 2×10-4, 4×10-4, 5×10-3) were prepared by a sol-gel route: Mixtures of tetra-methoxy-silane, Tb(NO3)3·5H2O, Al(NO3)3·9H2O, ethanol, and water were condensed at 60 °C using HNO3 as catalyst. The samples were sintered at 950 °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 = 5×10-3 is glasslike on the outer part showing onset of crystallization in the inner part of the sample. The samples with smallest dopant concentrations (x = 2×10-4, 4×10-4) contain pores. All samples luminesce from the 5D4 and 5D3 states of Tb3+ with excitation at 355 nm. The 5D3 luminescence increases with respect to the 5D4 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 5D3 and 5D4 luminescence.