Color bleaching and oxygen diffusion in a niobium phosphate glass

The color of niobium phosphate glasses ranges from clear to deep blue depending on their thermal history. In this study, a glass with the nominal composition (mole %) 23K(2)O center dot 40Nb(2)O(5)center dot 37P(2)O(5) was treated in oxidizing and reducing atmospheres at temperatures around T-g (730 degrees C) and the color at room temperature was analyzed using optical spectroscopy. Electron spin resonance (ESR) spectroscopy shows that the blue color is associated with the presence of Nb4+ ions, which oxidize to Nb5+ in the clear glasses. Bleaching kinetics were determined from a series of heat treatments below T-g in flowing oxygen, and analyzed with an diffusion model that indicated an activation energy of 150 +/- 31 kJ/mol. (C) 2013 Elsevier B.V. All rights reserved.