High niobium oxide content in germanate glasses: Thermal, structural, and optical properties

Niobium alkali germanate glasses were synthesized by the melt-quenching technique. The ternary system (90-x)GeO2-xNb(2)O(5)-10K(2)O forms homogeneous glasses with x ranging from 0 to 20 mol%. Samples were investigated by DSC and XRD analysis, FTIR and Raman spectroscopy, and optical absorption. Structural and physical features are discussed in terms of Nb2O5 content. The niobium content increase in the glass network strongly modifies the thermal, structural and optical properties of alkali germanate glasses. DSC, Raman and FTIR analysis suggest niobium addition promotes NbO6 groups insertion close to GeO4 units of the glass network. XRD analysis also pointed out that samples containing high niobium oxide contents exhibit preferential niobium oxide-rich phase after crystallization after heat treatment, which is similar to orthorhombic Nb2O5. Absorption spectra revealed high transmission range between 400 nm to 6.2 mu m, added to a considerably decreased hydroxyl group content as the addition of niobium in the alkali germanate network. The niobium oxide-rich phase crystallization process was studied and activation energy was determined, as well as nucleation and crystal growth temperatures and time for obtaining transparent glass-ceramics.