Adrian C. Wright

Neutron scattering from vitreous silica. V. The structure of vitreous silica: What have we learned from 60 years of diffraction studies?

Abstract A quantitative assessment is presented of the structural information derived from X-ray and neutron diffraction and small angle scattering investigations of vitreous silica, starting from the classic studies of Warren and co-workers in the 1930s. Particular points that are addressed include the regularity of the SiO4 tetrahedral structural units, the SiOSi angle distribution, the interpretation of the first peak in the diffraction pattern and the extent of the longer range density fluctuations as revealed by X-ray and neutron measurements at small scattering vectors. The use of structural modelling in the interpretation of the diffraction data is discussed and representative models are compared with experiment. The extent of the (dis)agreement of each model with experiment is compared quantitatively and the paper concludes by identifying the most accurate structural model of vitreous silica known to the author at the present time.

Neutron scattering from vitreous silica IV. Time-of-flight diffraction☆

Abstract The optimum configuration for a neutron time-of-flight diffractometer to study amorphous solids is discussed and it is concluded that measurements should be performed at relatively low scattering angles using a cold moderator. Good reciprocal space resolution is also important. Time-of-flight data for vitreous silica are combined with twin-axis data from part II (P.A.V. Johnson et al., J. Non-Cryst. Solids 58 (1983) 109) to give an accurate composite interference function and excellent real space resolution on Fourier transformation. The maximum scattering vector, Q max , is 45.2 A −1 . A peak fit to the resulting correlation function yields a mean SiO bond length of 1.608±0.004 A .

A neutron diffraction investigation of the structure of vitreous boron trioxide

Abstract An extended review is given of previous structural work on vitreous B2O3 which comes to the conclusion that the best structural model comprises a mixed random network of boroxol groups and independent BO3 triangles. The neutron scattering length for 11B is slightly greater than that for oxygen so that neutrons should be more sensitive than X-rays to the presence of boroxol groups. Results are presented from a neutron diffraction investigation of vitreous 11B2O3, using the Harwell linac pulsed neutron source, in which data have been obtained to high scattering vector values ( Q max =40 A −1 ) yielding good real space resolution. It is concluded that these data are consistent with a mixed boroxol group - independent BO3 triangle network having 60% of the boron atoms in boroxol rings.

Neutron scattering from vitreous silica II. Twin-axis diffraction experiments

Reactor twin-axis neutron diffraction data have been obtained for vitreous silica at five incident wavelengths and analysed to give the total correlation function T(r). A fit to the first SiO and OO peaks in the Gaussian approximation yields distances of 1.61 and 2.63 A respectively. The peaks are, however, asymmetric, the form of the asymmetry being qualitatively as predicted for anharmonicity in the SiO bond stretching modes. The results show that, in order to avoid significant static approximation distortions of the distinct scattering, measurements on vitreous silica must be limited to scattering angles less than 80°. The effect of temperature on the interference function is demonstrated by an additional measurement at liquid nitrogen temperature.

Neutron diffraction and exafs evidence for TiO5 units in vitreous K2O · TiO2 · 2SiO2

A combination of neutron diffraction and X-ray absorption spectroscopic techniques has been used to investigate the Ti coordination in vitreous K2O· TiO2 · 2SiO2. In the former case, the Tizz.sbnd;O bond lengths have been studied using the technique of isotopic substitution. XANES (X-ray absorption near-edge structure) data indicate that there is little or no evidence for a centre of inversion symmetry at the Ti site in the glass. TiO bond distances derived from neutron diffrraction and EXAFS (extended X-ray absorption fine structure) agree to within 0.03 A and it is concluded that the Ti is present as TiO5 tetragonal pyramids containing one non-bridging oxygen atom associated witth a titanyl bond (Ti=0 = 1.65 A). The bridging TiO bond length is 1.96 A and TiTi correlations occur at 3.4 A and 6.1 A.

A neutron diffraction investigation of the structure of vitreous zinc chloride

Abstract A survey is presented of the AX 2 glasses and the corresponding crystalline polymorphs. The relationship between ZnCl 2 and the other members of the series is discussed and it is predicted that the number of shortest path seven and higher-membered rings in the non-chalcogenide AX 2 glasses is likely to be small. A neutron diffraction investigation of vitreous ZnCl 2 using the D4 diffractometer at ILL Grenoble shows that the structure of vitreous ZnCl 2 comprises a distorted random close packed array of Cl − ions with the Zn 2+ ions in tetrahedral holes, arranged in such a way as to maximise corner sharing of the resulting ZnCl 4 2− tetrahedra at the expense of edge and face sharing. The inability of molecular dynamics simulations, using purely ionic potentials, to predict the ZnZn component correlation function is taken as an indication of covalent character to the ZnCl bond. The crystalline polymorph formed on devitrification of dry ZnCl 2 glass is δ-ZnCl 2 and not the α-form as previously reported.

Boroxol groups in vitreous boron oxide: new evidence from neutron diffraction and inelastic neutron scattering studies

Abstract Although the nuclear magnetic resonance (NMR) and neutron diffraction evidence for the majority of the boron atoms in vitreous boron oxide being involved in boroxol groups would seem to be irrefutable, nevertheless several recent papers have challenged this conclusion. The present paper, therefore, examines these claims in the light of neutron diffraction data, with much greater accuracy than those reported previously, and of neutron inelastic scattering results, optimised to give the highest possible energy transfer resolution in the region of the boroxol ring breathing mode. A fit to the interference function, Qi(Q) , indicates that the fraction, f, of boron atoms in boroxol groups is 0.80 ± 0.05, while the neutron weighted density of states exhibits a well resolved boroxol ring breathing mode of similar magnitude to the theoretical prediction for a structural model with f = 0.75. Both results confirm the conclusion of the earlier NMR and neutron studies.

Neutron and X-ray amorphography

Abstract A brief account is presented of the use of neutron and X-ray diffraction techniques in studying the structure of amorphous solids. An emphasis is placed on the exact nature of the information provided by diffraction measurements and on the correct methods of extracting structural parameters from experimental data. The use of modelling to investigate order beyond the basic structural unit is discussed, together with the need for accurate quantitative comparisons between models and experiment. Modern diffraction procedures are capable of yielding high quality data and it is concluded that, when used properly, these provide a stringent test of the various structural models proposed in the literature.

The structure of some simple amorphous network solids revisited

Abstract The amorphographic relationships between the structures of a number of single component amorphous network solids are discussed. New neutron diffraction data are introduced for several of the materials including vitreous P 2 O 5 which has not previously been studied by neutron diffraction. These data suggest that network connectivity is more important than structural unit stereochemistry in determining intermediate range order. Previous neutron data for amorphous Ge and vitreous SiO 2 are compared with bith random network and crystal based models and criteria are proposed for the minimum size of model required to accurately predict the shape of the first diffraction peak, which for a number of amorphous network solids is found to be Lorentzian. It is concluded that new computer modelling techniques are capable of generating models in good agreement with modern diffraction data and should considerably enhance present understanding of the structure of amorphous solids.

The comparison of molecular dynamics simulations with diffraction experiments

Abstract This Commentary discusses the need for a quantitative method of comparing molecular dynamics simulations and other structural models of amorphous solids with diffraction data and hence of assessing the relative merits of the various simulations and models proposed in the literature, most of which claim, erroneously, to be in good agreement with experiment. Using the molecular dynamics simulation of a recent Letter to the Editor [B. Vessal et al., J. Non-Cryst. Solids 159 (1993) 184] as an example, an Rχ, factor is advocated as a quantitative measure of the (dis)agreement with experiment, along the lines of that used in the refinement of diffraction data from polycrystalline powders. The Rχ factor of 9.1% for this simulation is compared with those for other well known structural models proposed in the literature.