Influence of heat treatment on the intensities of f–f transitions in lanthanide-doped sol–gel glasses

Abstract

Optical absorption spectra of monolithic silica sol–gel glasses incorporating the lanthanide complexes [Ln(bpy)₂]Cl₃, [Ln(phen)₂]Cl₃ and Ln(NO₃)₃ (bpy = bipyridyl, 2,2'-bipyridine, phen = 1,10-phenanthroline, Ln = Nd, Sm, Dy, Ho, Er) were recorded. The Judd–Ofelt theory was used to study changes in the coordination sphere before and after water was expelled from the glass matrix by heat treatment. The intensity of the hypersensitive transitions was found to increase significantly, which is reflected by an increase in the Ω₂ intensity parameter. The non-hypersensitive transitions are influenced by the native sol–gel matrix rather then by the type of ligands. The Ω₄ and Ω₆ parameters remain more or less constant upon heat treatment. In accordance with theoretical predictions the values of both Ω₄ and Ω₆ of the lighter lanthanides are larger than those of the heavy lanthanides.