Ruth Anderson

An x-ray diffraction and 31P MAS NMR study of rare-earth phosphate glasses, (R2O3)x(P2O5)1-x, x = 0.175-0.263, R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er

An x-ray diffraction and 31P MAS NMR study of rare-earth phosphate glasses of composition, (R2O3)xP2O5)1-x, where x = 0.175-0.263 and R = La-Er (except for Pm), is presented. The structures of these materials were investigated as a function of (a) rare-earth atomic number and (b) glass composition. The results show an increase in rare-earth coordination number from six to seven as the rare-earth ion increases in size. This effect is most evident for the rare earths, Ce, Pr and Nd, and appears to be independent of composition variation. The implications of sevenfold coordination in these glasses with respect to the possibilities of rare-earth clustering are discussed, as is the role of the incorporation of aluminium impurities in this regard. The increase in levels of cross-linking within the phosphate network, as a consequence of these small amounts of aluminium, is illustrated, as is the changing nature of the phosphate groups as a function of composition. The first reliable and quantitative parametrization of the second and third neighbour R-(O)-P and R-(OP)-O correlations is also given and the stability of the structures to strain when the glasses are drawn as fibres or exposed to different thermal conditions is described.

Changes in the Zr environment in zirconia–silica xerogels with composition and heat treatment as revealed by Zr K-edge XANES and EXAFS

X-ray absorption spectroscopy at the Zr K-edge is an important technique for probing the environment of Zr. Here it is applied to zirconia–silica xerogels with composition 0.07⩽x⩽0.40, where x is the molar ratio Zr:(Zr+Si). Reference samples of crystalline ZrO2, ZrSiO4, BaZrO3 and liquid Zr n-propoxide were also examined. New XANES (X-ray adsorption near edge structure) results are presented for zirconia–silica xerogels, and compared with previous EXAFS (extended X-ray absorption fine structure) results. For high Zr contents (x=0.4) there is a separate, amorphous ZrO2 phase, which before heat treatment is similar to Zr hydroxide, and after heat treatment at 750°C is similar to an amorphous precursor of tetragonal ZrO2. For low Zr contents (x=0.1) there is atomic mixing of Zr in the SiO2 network, and the environment of Zr is more similar to that in Zr n-propoxide compared to other reference samples. New in situ XANES and EXAFS results are presented for x=0.1 xerogels heated at 250°C. These clearly show that the Zr environment depends on ambient moisture in addition to heat treatment.

An extended x-ray absorption fine structure study of rare-earth phosphate glasses near the metaphosphate composition

A variable-temperature (79, 145, and 293 K) extended x-ray absorption fine structure study, using rare-earth L-III absorption edges, is reported for phosphate glasses doped with rare-earth elements (R, where R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) with compositions close to metaphosphate, R(PO3)(3). The results yield nearest-neighbor R-O distances that demonstrate the lanthanide contraction in a glassy matrix and an R-O coordination intermediate between 6 and 7 for ran-earth ions with smaller atomic number (Z) and 6 for rare-earth ions with larger Z, Thermal parameters show no significant changes in R-O distances or coordination numbers between 293 and 79 K. There is evidence of an R-P correlation between 3.3 and 3.6 Angstrom and the beginning of a second R-O correlation at approximately 4 Angstrom. No R-R correlations up to a distance of approximately 4 ii were observed.

Advanced physical characterisation of the structural evolution of amorphous (TiO2) x (SiO2)1−x sol-gel materials

Amorphous (TiO2)x(SiO2)1−x (x = 0.08, 0.18, and 0.41) sol-gel formed materials have been characterised by a combination of X-ray and neutron diffraction, infra-red and 29Si and 17O magic angle spinning NMR spectroscopy. This combination allows new insight into the fundamental structural role titanium additions play in silica, entering the network at x = 0.08 but largely phase separating at x = 0.41. At an intermediate value of x = 0.18 there is complex coordination behaviour with initially some Ti—O—Ti linkages forming and both TiO4 and TiO6 coordinations present. It appears that at 500°C for the x = 0.18 sample all titanium is present in Ti—O—Si linkages but that this situation is unstable on further calcination. The new information presented here is amalgamated with that from our previous EXAFS, XANES and SAXS on the same samples to produce the most complete view to date of the local and mesoscopic structural behaviour of the (TiO2)x(SiO2)1−x system produced by the sol-gel method.

An EXAFS study of silica-titania sol-gels

A series of samples, with up to 41 mol% TiO2, have previously been studied using 17 O magic angle spinning nuclear magnetic resonance (MAS NMR) X-ray diAraction and neutron diAraction. We report here the results of an extended X-ray absorption fine structure (EXAFS) experiment on these samples, carried out to probe more directly the coordination state of the titanium site in the dried gel and its evolution with heat treatment. At Ti contents small enough to ensure atomic homogeneity, the data are consistent with 4-fold oxygen coordination around Ti, as indicated by the 17 O MAS NMR spectra. At Ti contents large enough to bring about phase separation, the results show evidence of a structural change around the Ti atoms, with a larger Ti‐O distance compared with the results for the glasses with smaller TiO2 contents. ” 1998 Elsevier Science B.V. All rights reserved.

The effect of zirconia content on the structure of zirconia-silica xerogels as determined by x-ray and neutron diffraction and Zr K-edge EXAFS and XANES

We present combined x-ray and neutron diffraction, and extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) results on a single set of (ZrO2 )x (SiO2 )1-x xerogel samples. In these samples, there is improved homogeneity of Zr compared to our previous study, due to greater dilution of the precursor Zr n-propoxide in propan-1-ol. Structural parameters obtained from model fitting of the diffraction and EXAFS data are compared with those in reference compounds. A qualitative comparison of XANES spectra is also made. The results show that for x 0.3 there is phase separation of ZrO2 and the Zr environment is similar to that in monoclinic ZrO2 and Zr hydroxide. For x 0.2 there is no phase separation, the SiO2 network is distorted and the Zr coordination is similar to that in Zr n-propoxide, with some Zr-Zr clustering.

Reverse Monte Carlo modelling of Eu and Tb metaphosphate glasses

It is possible to make phosphate glasses (R2O3)x(P2O5)1ˇx with contents of rare-earth elements, R, close to the metaphosphate composition, R(PO3)3. These glasses are particularly stable to water, and have interesting optical, acoustic and magnetic properties. We have modelled Eu and Tb metaphosphate glasses using the reverse Monte Carlo (RMC) technique with X-ray diAraction data. The model building process must include additional constraints, and we have derived these from knowledge of metaphosphate structures, including metaphosphate crystals. The model building procedure was first to fit structural constraints alone, and then to fit the diAraction data while maintaining the structural constraints. The models constructed consist of chains of phosphate tetrahedra with Eu (or Tb) 6-fold coordinated to non-bridging oxygens (NBOs), and are in good agreement with the diAraction data. Thus the models provide a demonstration of validity of this structural description of rare-earth metaphosphate glasses. In addition, two diAerent values were used for minimum rare-earth separation, 3.9 and 5.4 A, based on two diAerent metaphosphate crystals, and more satisfactory models were obtained when using the shorter distance. ” 1998 Elsevier Science B.V. All rights reserved.

Inhomogeneities in acid-catalyzed titania–silica and zirconia–silica xerogels as revealed by small-angle x-ray scattering

The small-angle x-ray scattering (SAXS) technique was used to investigate inhomogeneities on the scale of 10 to 600 Angstrom in acid-catalyzed titania-silica and zirconia-silica xerogels. SAXS of (TiO2)(SiO2)(1-x) and (ZrO2)(x)(SiO2)(1-x) xerogels with x 0.3 showed the presence of phase-separated regions of metal oxide, which were initially amorphous and crystallized at higher temperatures. A (TiO2)(0.18)(SiO2)(0.82) xerogel that was not initially phase separated became phase separated after heat treatment at 750 degrees C due to reduced solubility of Ti in the silica network.