부산대학교 도서관

High energy X-ray diffraction (HEXRD) has been used to study the atomic structure of ([Ta.sub.2]O.sub.5).sub.x(SiO.sub.2).sub.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 [proportional to]1.8 A and 2.0 A). 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 [proportional to]3.3 A and [proportional to]3.8 A, 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 ([Ta.sub.2]O.sub.5).sub.0.25(SiO.sub.2).sub.0.75 sample. However, in general, the results indicate good mixing of the component oxides.