The coordination environment of 20-200 ppm Pt in yellowish glasses from the CaO-Al 2 O 3 -SiO 2 (CAS) ternary was studied using X-ray absorption fine structure spectroscopy at the Pt-L III edge. Analysis of the Pt-L III edge region suggests that Pt in these glasses is mainly tetravalent and sixfold-coordinated by O (with a mean Pt-O distance of 2.08+ or -0.02 Aa). No evidence for Pt (super 2+) or Pt (super 6+) was found in any of the glasses studied, suggesting that one can not derive valence information easily from solubility data. No second-neighbor contribution was observed around Pt (super 4+) O 6 polyhedra. However, bond-valence modeling suggest that these polyhedra are likely to bond mostly to (super [VI]) Ca (super 2+) , which should promote high positional disorder of second-neighbor cations around Pt. This particular bonding arrangement may explain the relatively high solubility of Pt in these relatively depolymerized melts, as CaPtO 3 -type units.
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Francois Farges, Daniel R. Neuville, and Gordon E. Brown
Universite de Marne-la-Vallee, Marne-la-Vallee, France
Institute de Physique du Globe de Paris, France
Stanford University, United States