INCORPORATION OF H-2 IN VITREOUS SILICA, QUALITATIVE AND QUANTITATIVE-DETERMINATION FROM RAMAN AND INFRARED-SPECTROSCOPY

Incorporation mechanisms of H-2 in silica glass were studied with Rama n and infrared (IR) microspectroscopy. Hydrogenated samples were prepa red at temperatures between 800 degrees C and 955 degrees C at 2 kbar total pressure. Hydrogen fugacities (f(H2)) were controlled using the double capsule technique with the iron-wustite (IW) buffer assemblage generating f(H2) of 1290-1370 bars corresponding to H-2 partial pressu res (P-H2) of 960-975 bars. We found that silica glass hydrogenated un der such conditions contains molecular hydrogen (H-2) in addition to S M and SiOH groups. H-2 molecules dissolved in the quenched glasses int roduce a band at 4136 cm(-1) in the Raman spectra which in comparison to that of gaseous H-2 is wider and is shifted to lower frequency. IR spectra of hydrogenated samples contain a band at 4138 cm(-1) which we assign to the stretching vibration of H-2 molecules located in non-ce ntrosymmetric sites. The Raman and IR spectra indicate that the dissol ved H-2 molecules interact with the silicate network. We suggest that the H-2 band is the envelope of at least three components due to the o ccupation of at least three different interstitial sites by H-2 molecu les. Both, Raman and IR spectra of hydrogenated glasses contain bands at similar to 2255 cm(-1) which may be due to the vibration of SM grou ps. Under the assumption that the reaction Si-O-Si + H-2 --> Si-H + Si -O-H describes adequately the 'chemical dissolution' of H-2 molecules, the SiH concentrations in our samples were determined and the molar e xtinction coefficient for the SM absorption band in the infrared (epsi lon(2255)(SiH)) could then be estimated to be 45 +/- 3 1/mol cm. The s olubility of molecular H-2 in our hydrogenated samples was determined using the IR absorption band at 4138 cm(-1) and the extinction coeffic ient given by Shelby [J. Non-Cryst. Solids 179 (1994) 138]. Samples qu enched with different cooling rates gave nearly identical Raman and IR spectra, suggesting that the chemical dissolution of hydrogen (SM and SiOH) can be quenched to room temperature without changing relative c oncentrations and that no exsolution of hydrogen occurred during the q uench. (C) 1998 Elsevier Science B.V. All rights reserved.