Espectroscopia e transferÃncia de energia em eidros La2O3-Nb2O5-B2O3

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

Glasses are amorphous materials with structures that can, in general, be described by a random non-periodic network model. Although for many glasses, mainly silicates, this model is appropriate, there are systems that present short and medium-range order (first and second coordination spheres) for which more sophisticated models are needed to describe their structure. The luminescence of nobate groups is well characterized for crystalline systems and it is strongly dependent upon the crystal structure. As a result, the presence of niobate luminescence in the La2O3-Nb2O5-B2O3 glass suggests a considerable degree of structural order. Based upon spectroscopy studies in niobate glasses and energy transfer doped samples with lanthanide ions, and of the comparison with the luminescence of crystalline models, we have proposed a structural model for La2O3-Nb2O5-B2O3 glasses that explains their luminescence properties, particularly, with respect to the chemical composition. We have also performed theoretical calculations of the electronic states for the crystalline structure K2NbOF5 in order to establish the dependence of the niobate luminescence structural distortions. We have concluded that the luminescence of niobate can be attributed to the charge transfer state localized in the Nb��O bond, and the transition energies follow the particle in a box model for the distortions of the Nb��O bond. The formation of chains of corner-sharing NbO6 groups leads to electronic delocalization and causes a red shift in the spectra (towards smaller energies transitions). On other hand, edge-sharing NbO6 group or niobil groups (one shorter Nb��O bond), leads to excited state localization (self-trapped excitons) and have spectra shifted towards higher energies (blue shift) and larger quantum efficiencies. Even in samples with small concentrations of niobium, at 20 mol% of La2O3, pairs of edge-sharing NbO6 groups occur and the luminescence is typical for this structure. The increase of La2O3 concentrations at 25 mol% seems to disrupt these chains of niobate edgesharing into corner-sharing off-center chains. This leads to structural changes that are reflected into the luminescence properties. At concentrations larger than 25 mol% it appears that part of lanthanum segregate in a different dominium, although this has not been perceived as a phase separation in the sample. This segregation leads to an effective decrease of the La concentration around the niobate group that favors the formation of edge-sharing pairs, which are characterized in the luminescence spectra

ASSUNTO(S)

eidros transferÃncia de energia espectroscopia quimica

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