A high energy X-ray diffraction study of sol-gel derived (Ta2O5)(x)(SiO2)(1-x) glasses (x=0.05, 0.11 and 0.25) - elucidating the role of tantalum in silica

High energy X-ray diffraction (HEXRD) has been used to study the atomic structure of (Ta2O5) (x) (SiO2)(1-x) (x = 0.05, 0.11 and 0.25) xerogels; the direct interpretation of the resultant data has been augmented using reverse Monte Carlo (RMC) modelling. For the first time in this type of material, two Ta-O correlations have been identified (at similar to 1.8 angstrom and 2.0 angstrom). The RMC modelling approach explicitly used MAS-NMR data to define its constraints; when combined with HEXRD data, it helps to confirm the more directly determined total coordination of five for the two Ta-O correlation distances and suggests Ta center dot center dot center dot Si and Ta center dot center dot center dot Ta coordination distances of similar to 3.3 angstrom and similar to 3.8 angstrom, respectively. The O center dot center dot center dot O and Si center dot center dot center dot Si distances and coordination numbers associated with the host silica network suggest that the Ta (V) is acting as a network modifier. The way in which the Ta-O correlations are affected by composition and calcination temperature suggest some phase separation in the (Ta2O5)(0.25)(SiO2)(0.75) sample. However, in general, the results indicate good mixing of the component oxides.