The structure of TiO2-SiO2 sol-gel glasses from neutron diffraction with isotopic substitution of titanium and O-17 and Ti-49 solid-state NMR with isotopic enrichment

Neutron diffraction with Ti-46 and Ti-48 stable isotopes and isotope-enriched O-17 and Ti-49 MAS NMR have been used to characterize the structure of (TiO2)(x)(SiO2)(1-x) sol-gel glass as a function of composition (x = 0.08, 0.18, and 0.41) and calcination temperature (T = 250, 500, and 750 degreesC). The results reveal the first direct observation of two Ti-O distances in a homogeneous (TiO2)(0.18)(SiO2)(0.82) sol-gel derived glass. In the sample heat treated at 250 degreesC, the Ti occupies a distorted octahedral environment similar to that found in the mineral ramsayite with four Ti-O bond lengths of around 1.89 Angstrom and two close to 2.11 Angstrom. After heating to 750 degreesC, two shorter bond distances are observed: a short distance at 1.81 Angstrom due to tetrahedrally coordinated Ti and a longer distance of 1.94 Angstrom due to a minority species of octahedrally coordinated Ti. The (TiO2)(0.08)(SiO2)(0.92) sample exhibits similar behavior. After heating to 250 degreesC, two Ti-O distances are observed at 1.84 and 2.10 Angstrom consistent with the presence of both tetrahedral and distorted octahedral titanium. Heating to higher temperature (500 or 750 degreesC) leads to the presence of only a single Ti-O distance at 1.82 Angstrom consistent with all the titanium being substituted in tetrahedral sites within the silica network. O-17 NMR on samples at 45 atom % isotopic enrichment is very sensitive to phase separation. The (TiO2)(0.18)(SiO2)(0.82) sample exhibits only a very small amount of phase separation in the form of a weak but nevertheless definite Ti-O-Ti signal. More significant phase separation of TiO2 can be observed in the O-17 NMR spectrum from the (TiO2)(0.41)(SiO2)(0.59) sample after heating at both 250 and 5 00 degreesC. Ti-49 NMR spectra are quite broad in all samples but some trends in line width and position are discerned. The results presented here are consistent with, but greatly extend, previous XRD, O-17 and Si-29 MAS NMR, XANES, and EXAFS studies of these materials