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Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P138
Na2O - Fe2O3 - SiO2 glasses: Inhomogeneous distribution of Fe in channels
Florian Kargl 1,2 , Andreas Meyer 2 , Michael Marek Koza 3
1 Department of Applied Science, Chalmers University of Technology, 41296 Göteborg,
Schweden – 2 Deutsches Zentrum für Luft und Raumfahrt, Institut für Raumsimulation,
51170 Köln – 3 Institute Laue-Langevin, 38042 Grenoble, Frankreich
We report on the investigation of ternary sodium iron-silicate glasses by means of neutron
spectroscopy and diffraction. Being of outstanding importance for geosciences,
iron bearing silicate glasses and melts show a number of unique features [1]. Adding
Fe2O3 to a binary alkali silicate melt leads to an increase in melt viscosity. This increase
depends not only on the amount of Fe2O3, but also on the conditions of synthesis.
For instance, under normal conditions of synthesis Fe is predominantly built into the
structure as Fe 3+ . Depending on melt composition, temperature, and atmosphere Fe
can also exist in the reduced form Fe 2+ .
We demonstrate that a partial Fe-Fe structure factor can be directly obtained in these
multicomponent system. Neutrons scattered on Fe 3+ couple to its unpaired electron
spin moment. Quasielastic neutron scattering allows to separate nuclear from magnetic
scattering contributions in sodium iron silicates. The results were corroborated by
experiments using polarization analysis. The partial Fe-Fe structure factor determined
by means of magnetic scattering features a correlation peak around 0.9 ˚A −1 . A
correlation peak in the nuclear elastic structure factor observed at the same position
in binary alkali silicate melts was shown to reflect a network of percolation channels
for fast alkali ion diffusion [2]. The addition of a second network forming component,
Al2O3, leads to a drastic increase in viscosity and a disruption of the channel structure.
This is evidenced by vanishing of this correlation peak in the nuclear elastic structure
factor [3]. Considering the total nuclear structure factor consisting of ten different
partial structure factors the position of Al with respect to the other components cannot
be unambiguously determined. In the sodium iron-silicates the position of the correlation
peak is Fe concentration independent. However, the peak height scales with the
total Fe3+ content. The experiments indicate an inhomogeneous distribution of Fe.
They further suggest that the Fe atoms are located on Na equivalent positions rather
than on those equivalent to Si.
[1] Y. Bottinga, D. B. Dingwell, and P. Richet, edts., 6th Silicate Melt Workshop,
Chem. Geol. 174, (2001) 1; Y. Bottinga, D. B. Dingwell, P. Richet, and M. Toplis
edts., 7th Silicate Melt Workshop, Chem. Geol. 213, (2004) 1.
[2] A. Meyer, J. Horbach, W. Kob, F. Kargl, and H. Schober, Phys. Rev. Lett. 93,
(2004) 027801.
[3] F. Kargl and A. Meyer, Chem. Geol. 213, (2004) 165.