Coloration and Decoloration of Tungsten Phosphate Glasses by Heat Treatments at the Temperature Far below T_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 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 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 cm2/s.

Catalysis and proton-conduction of novel phosphate glasses

“…content dependence of the change, DP H2 , in the hydrogen partial pressure in room-2 measured by exposing glass specimens to water vapor at 500°C. The inset shows the time dependence of P H2 in room-2 for a specimen of 30Na 2 …”

“…Recently, it was reported that some phosphate glasses containing tungsten oxide decompose water vapor and absorb hydrogen [2,3]. Spectrometric studies on these glasses showed that the glasses had absorption bands in the IR and visible (600-650 nm) regions, which were assigned to the vibration mode of O-H and the absorption by W 5+ ion, respectively.…”

Mixed alkali effect in physical and optical properties of Li2O–Na2O–WO3–B2O3 glasses

“…For a value of x ≤ 0.33, glasses are transparent [24], which can be used for modeling typical glass applications such as sensors for the investigation of structural aspects of the amorphous materials [29,13]. Tawarayama et al reported that during heat treatment, the 30Na 2 O-10BaO-30P 2 O 5 -20Nb 2 O 5 -10WO 3 glass is permeable to hydrogen, and as monoatomic hydrogen passes through the glass, it is reducing tungsten ions from W 6+ to W 5+ [30,31]. They proposed that the fast coloration requires fast diffusion of electrons and protons introduced into the glass.…”

Hydrogen permeation of tungsten phosphate glass thin films

“…Tungsten phosphate glasses react with hydrogen and water vapor at the ambient temperatures below the glass transition temperature (T g ) and W 6+ ions in the glasses are reduced to W 5+ [7,8]. The reduced tungsten ions, W 5+ , show strong intraionic and intervalence optical absorptions in the visible to near infrared region and the color of the glass changes to dark blue.…”