Structural and Optical Properties of Erbium and Ytterbium Codoped Germanoniobophosphate Glasses

A series of glasses with compositions of 20Na(2)O-30Nb(2)O(5)-(5-y-z)Al2O3-30P(2)O(5)-(15-x)TiO2-xGeO(2)-yEr(2)O(3)-zYb(2)O(3), where x = (0; 5; 10; 15), y = (0; 1), z = (0; 2) mol%, were investigated with respect to their structural, optical, and luminescence properties. The coordination of the germanium(IV) ion is normally reported as being mainly tetrahedral. However, results of this study suggest that the germanium(IV) ion may have an octahedral coordination and that TiO2 is substituted. This proposition can be done mainly by P-31 MAS-NMR spectroscopy, which spectra show predominantly pyrophosphate chains in the different glasses, without changes in their polymerization after substitution. A similar coordination of germanium can also be identified by the photoluminescence behavior of the different codoped samples, which shows similar erbium(III) emission decay lifetimes (5 ms), and Judd-Ofelt intensity parameters. It was found that the upconversion emission process involved 1.5 photons. Regarding the thermal behavior, it is noted that the glasses containing higher proportions of GeO2 exhibit higher thermal stability and are therefore more resistant to devitrification when compared to compositions containing more TiO2.