Coloration and Decoloration of Tungsten Phosphate Glasses by Heat Treatments at the Temperature Far below Tg under a Controlled Ambient

Reversible coloration−decoloration of tungsten phosphate glasses was observed by heat treatments at temperatures between 300 and 500 °C which were far below the Tg (570 °C) under the ambient with controlled partial pressure of hydrogen and/or water. The color center responsible for the coloration was found to be an electron-trapped W5+. The assignment was done by detecting an anisotropic ESR singlet of g = 1.7 and broad optical absorption bands extending from the visible to the near-infrared region. The charge compensator of W5+ was found to be H+. Mechanisms of the coloration by heat treatments under an ambient with the hydrogen-bearing species were proposed to be (1) decomposition of molecular water or hydrogen at the surface of the glasses, (2) selective dissolution of the generated hydrogen atom by the reaction with W6+ ions to form the W5+/H+ redox pair in the glasses, and (3) diffusion of the charge-compensating H+ into the glass. The decoloration process was expressed by the reverse reactions. The ...