Neutron diffraction and exafs evidence for TiO5 units in vitreous K2O · TiO2 · 2SiO2

Pressure-induced coordination change of Ti in silicate glass: a XANES study

“…(The 15 kbar sample disintegrated during the quench such that no suitably large fragments remained for accurate density determination.) the 5-coordinated Ti in glass by Yarker et al (1986) is in agreement with the FWHM value for our room pressure spectrum and supports our interpretation of an average CN of 4.8. The increase of FWHM values in our set of spectra suggests therefore an increase of the octahedral component with pressure.…”

“…If the pre-edge peak P energy position is considered as the most reliable measure of oxidation state (Waychunas, 1987) the samples show an energy shift of 4.5-4.7 eV ( Table 1) relative to Ti metal. These values are consistent with the presence of tetravalent titanium but it is interesting to observe that they are much higher than those found for crystalline compounds with both tetrahedral (3.5 eV, Ba2TiO4: Yarker et al 1986) and octahedral coordination (2.0-3.5 eV : Waychunas 1987;Lopez et al 1993). Instead, they are consistent with the values found for glasses (3.8-4.6eV: Greegor etal.…”

“…The regression line calculated as , where CN is the coordination number and IP is the normalized intensity of peak P (R=0.95), has been used for obtaining average coordination numbers for the samples quenched from high pressures. The use of this approximation is supported by comparison with glasses whose CN has been independently determined and found to be in good agreement with our calculated values Yarker et al 1986). The application of a relationship obtained on crystalline compounds to glasses, appears perhaps suspect at first, but becomes less so when one considers that the resonance we are considering (peak P) is mostly due to interactions with the first coordination shell and it is therefore not affected by longrange order structural effects like other XANES resonances.…”

“…From the comparison with the standards, the glass at room pressure, whose pre-edge peak intensity (IP) is Yarker et al 1986). Figure 1 shows the variation of the XANES spectra with pressure, up to 30 kbar.…”

Environment of titanium and aluminum in a magnesium alumino-silicate glass

“…All methods used to determine the coordination number distribution for Ti atoms agree on the predominance of [5] Ti, with the coexistence of [4] Ti and [6] Ti in similar amounts. This is an important difference with other alkaline or alkaline-earth silicate glasses (see [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]) that usually present a strong preference for [5] Ti with minor amounts of [4] Ti. This corresponds to several structural positions for titanium atoms within the glass structure, network former or either network modifier.…”