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

Abstract

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 ∼1.8 Å and 2.0 Å). 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···Si and Ta···Ta coordination distances of ∼3.3 Å and ∼3.8 Å, respectively. The O···O and Si···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.