Influence of melting and annealing conditions on the optical spectra of a borosilicate glass doped with CoO and NiO

Abstract

In the high-viscosity borosilicate glass (NBS2) doped with 0.3 mol% CoO or NiO quenching resulted in a freeze-in snap-shot of the glass structure within the dopants' transformation process from their high-temperature tetrahedral coordination to the octahedral form normally present in this glass at room temperature. In this transitional state the octahedral, tetrahedral and a third pseudotetrahedral transitional coordination are simultaneously present. The optical spectra of the doped glasses are discussed in relation to the different melting and cooling conditions applied. Quenched glasses were also tempered on a heating table, which permitted to take the optical spectra at each temperature step. In contrast to Co2+, Ni2+ has a strong octahedral preference. Thus for NiO-doped NBS2 glass tempering or annealing always results in relaxation into the octahedral coordination. For Co2+, which is also octahedrally coordinated in the annealed NBS2, tempering of the quenched glass leads to a relaxation into octahedrally and tetrahedrally coordinated Co2+. These structural changes are especially strong when the applied temperatures lie 150 to 200 °C above Tg of the NBS2 glass where also the viscosity-temperature curve implies structural changes within the glass matrix.