Roger N. Sinclair

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.

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.

Neutron scattering from vitreous silica: III. Elastic diffraction

Abstract The differences between elastic and total neutron diffraction experiments on amorphous solids are discussed quantitatively using data for vitreous SiO2. It is shown that in the former case early peaks in the real space correlation function T(r) are broader due to the inclusion of the effects of acoustic phonons and that for vitreous silica the atomic thermal vibrations, relative to the SiO bond direction, are approximately isotrpic. The two techniques yield the same SiO bond length, indicating that recoil corrections are unnecessary in total diffraction studies of amorphous solids. Supplementary constant Q scans across ω-Q space at high Q are qualitatively in agreement with a simple twin Gaussian model for the scattering law S(Q, ω), which may prove useful in studying the effects of static approximation distortions.

Effect of preparation method on the structure of amorphous solids in the system AsS

Abstract A brief summary is given of previous neutron and X-ray diffraction data for vapour-deposited and bulk vitreous AsSx. The structure of the vapour-deposited material contains As4S4 molecular fragments. On the other hand an analysis of the first diffraction peak for vitreous AsSx and GeSe2 indicates that the evidence for a conventional layer structure in bulk chalcogenide glasses is much less certain.

The structure of vapour-deposited arsenic sulphides

Abstract High resolution X-ray and neutron correlation functions have been obtained for arsenic sulphide bulk glass and vapour-deposited films by making measurements to high momentum transfers. These data are compared with a vatiety of models and it is concluded that the structure of the films is dominated by the presence of As4S4 molecules in the vapour phase, which results in more As-As bonds than the minimum required by stoichoimetry. The extent to which these As4S4 molecules polymerise in the as-deposited film is unclear, but depends strongly on preparation conditions.