G.N. Greaves

EXAFS and the structure of glass

The relevance of EXAFS for studying the structure of glass is reviewed. Examples are taken from recent work on SiOx films, soda silica glasses, calcium and iron containing mineral glasses and amorphous biogenic phosphates. The sensitivity of the environment of modifiers to the morphology of the glass network is stressed. The concepts underlying a modified random network (MRN) are introduced and are discussed in relation to the rheological and diffusion properties of glasses.

Computer simulation of sodium disilicate glass

Extensive molecular dynamics simulations have been performed on sodium disilicate glass Na2Si2O5, using realistic empirical interaction potentials. The structure of the simulated glass agrees reasonably with experimental diffraction data. Analysis of this structure is performed by a number of different methods and the results provide support for the modified random network model. Quantitative values of the sodium diffusion coefficient over the temperature range 1000–2000 K yield a diffusion activation energy which is close to the experimental value.

Colour centres in vitreous silica

Abstract Colour centres in vitreous silica are discussed in. terms of dangling bond defects on silicon and oxygen sites. Because of the large cation-anion electronegativity difference it is argued that neutral singly occupied states will be unstable, decomposing into empty positively charged silicon sp3 states and doubly occupied negatively charged oxygen 2p states. The energy levels for these states will be separate from the band edges and will give rise to features in the tail of the optical absorption edge. Following ionizing radiation, electrons and holes will be trapped at these defects converting them to neutral metastable singly occupied silicon and oxygen states. In line with current models for chalcogenide glasses, considerable distortion of the lattice is expected and this will lead to absorption bands well below the optical absorption edge. Intense radiation—particularly particle radiation—will increase the total number of defects–both charged and neutral. The absorption and luminescence data f...

Intrinsic and modified defect states in silica

Abstract Starting from a point defect involving a separated trivalent silicon and a non-bridging oxygen the optical properties of colour centres in pure silica are analysed. It is proposed that the silicon and oxygen states are charged in the annealed glass and that these are the precursors of the paramagnetic states produced with ionising radiation. A dipole centre in which the two defect ions share a common bond is described. This is used as a basis for analysing the colour centre properties of modified silica — the aim being to obtain a comprehensive picture of point defects in amorphous silicate networks. The effects of common impurities like water present in the different commercial forms of silica are also discussed.

EXAFS, glass structure and diffusion

Abstract The application of X-ray absorption spectroscopy (XAS) to the study of diffusion properties of oxide glasses and glass surfaces is considered. Mixed alkali silicates, the chemical resistance of iron-lead-phosphate glasses and the surface structure of uranium in leached borosilicate glasses are discussed. Results are interpreted using the modified random network (MRN) model. The importance of combining XAS with diffusion measurements and structural modelling is stressed if the interdependence of structure and ionic transport is to be properly elucidated.

A structural basis for the corrosion resistance of lead-iron-phosphate glasses: An X-ray absorption spectroscopy study

Abstract Lead metaphosphate glass is characteristically prone to aqueous attack but becomes chemically resistant on the addition of iron oxide. We have measured the K-edge extended X-ray absorption fine structure (EXAFS) of Fe and the LIII-edge EXAFS of Pb and find the atomic environments of the two sites to be very different. Whereas Fe is octahedrally coordinated with an oxygen distancce of 1•9(5) A, the coordination of Pb is 8 (±1) and the sites display considerable disorder with an average lead—oxygen distance of 2•4(7) A. X-ray absorption near-edge structure (XANES) measurements of the P K-edge confirm the presence of phosphate as (PO4)n, chains. These results suggest that, by pinning the structure, Fe sites not only shorten the average polyphosphate chain length but also block the percolation of Pb ions. Both effects increase the chemical resistance of the structure to aqueous attack and also reduce the tendency of these glasses to crystallize.

Extended X-ray absorption fine-structure spectroscopy study of photostructural changes in amorphous arsenic chalcogenides

Abstract Extended X-ray absorption fine-structure spectroscopy (EXAFS) measurements have been performed on amorphous arsenic sulphide, in both bulk-glass and thin-film forms, in order to probe the changes in local order accompanying the photostructural changes which occur in such materials. The irreversible photo-induced changes occurring in the thin films are associated with changes in the first coordination shell; these are ascribed to the conversion of As-As bonds to As-S bonds. In contrast, no significant changes in the first coordination shell, as revealed by EXAFS, appear to accompany the reversible photo-induced changes observed in glassy As2S3. Instead, changes in the region of the second shell are observed, and these are ascribed to either an increase in (bond-angle) disorder, or to an increase in inter-molecular distances, occurring on illumination.

EXAFS for studying corrosion of glass surfaces

Abstract The technique of glancing angle EXAFS is introduced and applied to the study of aqueous corrosion at the surface of sodium borosilicate glass. In particular, the structural roles of iron and uranium are compared at loadings typical for nuclear waste disposal. The structural chemistry observed at the surface is rich in detail and adds considerably to the qualitative picture for the leaching of oxide glasses obtained from conventional surface analytical probes. Both metald migrate to the surface as corrosion progresses, iron more rapidly than uranium. There is evidence for mineralisation of the modifiers at the surface following leaching and, in the case of uranium, as a result of polishing. Indeed the uranyl configuration of U 6+ is particularly sensitive to changes just beyond the oxygen ligand. Wet and dry surfaces can be distinguished and square planar clustering can be identified. The sequence of events is interpreted in terms of ion migration and water infusion taking place along percolation channels within the borosilicate network.

Photodissolution of silver in arsenic sulphide films — an exafs study

Abstract Silver K-edge EXAFS measurements have shown that the photodissolved silver site in arsenic sulphide films is very similar to that in the bulk ternary glasses of the general composition Ag x (As 2 S 3 )( 1−x ). Neither the films nor the glasses show a marked resemblance to any of the simple crystalline sulphides. In particular silver is 3-fold coordinated to sulphur without any pronounced Ague5f8Ag or Ague5f8As correlations. In those cases where a surfeit of silver has been added the EXAFS of the photodoped films exhibit a second neighbour peak which is interpreted as an incomplete dissolution of silver.

Photostructural changes in bulk chalcogenide glasses: An EXAFS study

EXAFS studies of reversible photostructural changes in bulk As and Ge chalcogenide glasses have been performed. The compositions studied were Ge33Se67, Ge40Se60, Ge40S60, As40S60 and As43S57. The magnitude of the photostructural change in both germanium and arsenic systems is larger in off-stoichiometry samples. In the Asue5f8S system the structural change is characterised by an increase in the Asue5f8Sue5f8As bond angle, whereas in the Geue5f8S (Se) systems a bond-breaking mechanism dominates. These data are consistent with neutron scattering and electron spin resonance data.