Medium-range order around titanium in a silicate glass studied by neutron diffraction with isotopic substitution

The structure of a silicate glass of composition K₂O·TiO₂·2SiO₂ has been reinvestigated by neutron diffraction with isotopic substitution of Ti. These data confirm Ti to be 5-coordinated within a square-based pyramid. The second difference function, which gives directly the Ti-Ti distribution, shows a first Ti-Ti distance at 3.5 {}, with second and third Ti neighbors at 6 {} and 8 {}. These values correspond to a nonhomogeneous distribution of Ti in the glass structure and are consistent with corner-sharing TiO₅ pyramids. This explains how this peculiar TiO₅ site, with one nonbridging O at 1.68 {} and four O at 1.96 {}, allows Ti to play both a former and a modifier role in the glass structure. The environment of potassium is consistent with a charge compensating role. The presence of Ti-enriched regions in the glass, which is in agreement with recent reverse Monte Carlo simulations, is favored by the potassium atoms acting as charge compensators around the short titanyl bond.