Coloration and Decoloration of Tungsten Phosphate Glasses by Heat Treatments at the Temperature Far below <i>T</i>_g under a Controlled Ambient

Abstract

Reversible coloration−decoloration of tungsten phosphate glasses was observed by heat treatments at temperatures between 300 and 500 °C which were far below the T g (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 W 5+ . 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 W 5+ 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 W 6+ ions to form the W 5+ /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 rate of the coloration−decoloration was found to be determined by the diffusion of the atomic hydrogen. The diffusion constant of the atomic hydrogen at 500 °C in the glass was found to be as large as 1.5 × 10 - 6 cm 2 /s.