An account is given of recent neutron diffraction studies of a number of different network glass forming systems using the ISIS pulsed neutron source. The glasses investigated comprise phosphates and silicates and the interrelationship between their various network structures is discussed. The samples studied include a single component glass and binary systems, involving either a network modifier or a conditional glass former, and the data have been extended to high values of the scattering vector magnitude, Q, to achieve high real space resolution. Peak fitting techniques are employed to extract the detailed geometry of the basic structural units and to investigate the distortions caused by the introduction of network modifiers. The data obtained are compared to various structural models and computer simulations to study the intermediate range order and to test current theories concerning the role of network modifiers in binary systems. These comparisons correctly include the necessary peak functions which define the experimental resolution in real space and are made on a quantitative basis by the use of an appropriate reliability factor.

A neutron diffraction investigation of the structure is reported for four glasses in the Na O-2-B2O3 -SiO2 system and one in the Na2O-B2O3 system. Measurements have been made to high values of the scattering vector magnitude, Q, in order to obtain high real space resolution. Of primary interest is the fraction, x(4) , of four -fold coordinated boron atoms present at each composition and the bond lengths within the borate and silicate structural units. These have been extracted using peak fitting techniques and compared both with NMR results and with the predictions of a model based on earlier NMR data. It is concluded that, within the experimental uncertainty, the values of x(4) are in agreement with the NMR results but at variance with those predicted by the model.

The neutron diffraction isotopic substitution technique is employed to investigate the environment of Fe3+/Fe2+ cations in a sodium borosilicate glass matrix of composition 0.210Na(2)O.0.185(11)B(2)O(3).0.605SiO(2). The neutron diffraction data were obtained using the D4c diffractometer at the Institut Laue-Langevin (ILL; Grenoble, France), and were recorded for three samples; the base glass, the base glass incorporating natural Fe2O3 (12 mol%) and a similar glass containing Fe2O3 enriched in Fe-57. The data are Fourier transformed to yield the real space total correlation function, T(r), and the first co-ordination shells of the Fe3+/Fe2+ cations are investigated via a peak fit to the isotopic difference correlation function Delta T-Fe(r). It is concluded that the iron is mainly present as Fe3+ cations, both tetrahedrally and octahedrally co-ordinated by oxygen atoms, plus a small fraction (0.07 +/- 0.01) of Fe2+ cations in octahedral co-ordination. The Fe3+ tetrahedral fraction is 0.45 +/- 0.10, and appears to exist as Fe empty set (-)(4) structural units incorporated into the network of silicate chemical groupings, with their negative charge being balanced by Na+ network modifying cations. The remaining Fe3+ cations (fraction 0.48 +/- 0.10) are thought to be predominantly octahedrally co-ordinated and associated with BO33- orthoborate anions in FeBO3 chemical groupings, which become non-stoichiometric due to the reduction of some of the Fe3+ cations to Fe2+.

Twin-axis neutron diffraction experiments have been performed to study structural aspects of the mixed-modifier effect for six ternary di-pentaborate glasses of composition M2O.M'O-2.5B(2)O(3) (M/M'=Li/Na, Li/Rb, Na/K, Na/Rb, Na/Cs, and Na/Ag). In reciprocal space, the method of additivity is employed to deduce information concerning the thickness/size of the network cages containing the network-modifying cations, and the zero-Q limit of the diffraction pattern, 1(0), to investigate the extent of the nanoheterogeneity in the glasses. The fraction of 4-fold co-ordinated boron atoms, x(4), is obtained from the area under the first (B-O) peak in the real-space total correlation function, T(r) and, zvith the exception of vitreous Na2O.Cs2O.5B(2)O(3) and Na2O.Ag2O.5B(2)O(3), no evidence is found to indicate the presence of a significant fraction of non-bridging oxygen atoms. An interpretation of the neutron diffraction data, in terms of a thermodynamically-based structural model for the network-modifying cation conductivity and mixed modifier effect in borate glasses, is presented in an accompanying paper (pp. 98-107).

A neutron diffraction investigation has been performed to investigate the effect of the network modifying cation fraction, x(M), on the structure of a series of binary sodium, rubidium and caesium borate glasses. The first peak in the corrected, normalised diffraction pattern, I(Q), is employed to estimate the distance between the network modifying cations occupying adjacent network cages, and the fraction of 4-fold co-ordinated boron atoms, x(4), is calculated from the area under the first (B-O) peak in the real-space correlation function, T(r). Up to the di-pentaborate composition, 2M(2)O center dot 5B(2)O(3) (M=Na, Rb or Cs), there is no evidence, within the experimental uncertainty, for significant numbers of nonbridging oxygen atoms. The fact that the first diffraction peak for the rubidium and caesium borate glasses remains sharp at low x(M) indicates that their structure is nanoheterogeneous, in agreement with the chemical structure predicted from thermodynamic modelling. The structure of the invert glass 2.13Rb(2)O center dot B2O3 includes tetrahedral borate structural units with nonbridging oxygen atoms, whereas that of vitreous 7Na(2)O center dot 3B(2)O(3) exhibits carbonate retention.

A neutron diffraction investigation has been performed to investigate the effect of the network modifying cation size and charge on the structure of a series of binary borate glasses of composition 2M(2)O center dot 5B(2)O(3)(M=Li, Na, K, Rb, Cs & Ag) and 2MO center dot 5B(2)O(3) (M=Ca & Ba). The diffraction patterns were recorded using a twin-axis diffractometer, and Fourier transformed to yield the real-space correlation function, T(r). The first peak in the corrected, normalised diffraction pattern, I(Q), is employed to estimate the spacing between the network-modifying cations occupying adjacent network cages, and the fraction of four-fold co-ordinated boron atoms, x(4), is obtained from the area under the first (B-O) peak in T(r). With the exception of 2CaO center dot 5B(2)O(3), and within the experimental uncertainty, no evidence is found to indicate the presence of significant numbers of nonbridging oxygen atoms. The way in which the distribution of superstructural unit species present varies with the network modifying cation is investigated by comparison with theoretical calculations based on the model of associated solutions. Finally, conclusions are drawn in respect of the first M(O) co-ordination shell and the important role of both the network modifying cation and superstructural units in determining the structure of binary alkali and alkaline earth borate glasses.

Iron phosphate (Fe2O3-P2O5) glasses with a high Fe2O3 content are particularly interesting in that they exhibit short range antiferromagnetic (speromagnetic) ordering at low temperatures. Neutron scattering techniques have been employed to investigate the atomic and magnetic structure and dynamics of four iron phosphate glasses, as a function of their nominal composition between 30 and 44 mol% Fe2O3, and the data are compared with earlier structural models in which the Fe3+ cations are either tetrahedrally or 6-fold co-ordinated by oxygen. Fe2+ cations are also found to be present in 6-fold (trigonal-prismatic and/or octahedral) co-ordination. Neutron magnetic diffraction experiments yield information concerning the Fe Fe distances and confirm the speromagnetic nature of the magnetic ordering, whilst a measurement of the magnetic inelastic scattering from vitreous 40Fe(2)O(3)center dot 60P(2)O(5) reveals the presence of dispersive magnetic excitations centred on the first magnetic diffraction peak at 0.82 angstrom(-1). It is concluded that all four glasses have structures that are much more complicated than the above models would suggest and include Fe3+ cations in both tetrahedral and octahedral co-ordination. A comparison with the structure of the corresponding crystalline phases suggests that the glasses are nanoheterogeneous, with FePO4-like regions (alternating Feempty set(4)(-) and Pempty set(4)(+) corner sharing tetrahedra) and those having a more typical phosphate chain structure incorporating Fe2+/Fe3+ network modifying cations.

A neutron diffraction investigation has been performed of the structure of four chalcogenide glasses, using the D4c diffractometer on the high-flux reactor at the Institut Laue-Langevin (ILL). Vitreous Ge3As52S45 is shown to have a structure based on As4S3 molecules, whereas that of its selenide analogue, vitreous Ge3As52Se45, involves far fewer As4Se3 molecules. Two As2X3 reference glasses (X = S or Se) have purely network structures. As–X, and As–As bond lengths have been extracted, together with their associated co-ordination numbers, from peak fits to the real space correlation functions, T(r), and suggest that all four glasses are chemically ordered; i.e. that they contain the maximum possible number of As–X (and Ge–X) bonds. The molecular nature of vitreous Ge3As52S45 is evident from the enhanced first diffraction peak relative to the As2S3 reference glass, and has been further investigated by comparing its interference function, Qi(Q), with that expected for an isolated As4S3 molecule. On the other hand, the second diffraction peak for vitreous Ge3As52S45 is not reproduced by a random orientation molecular model, indicating that there is orientational correlation between adjacent molecules, and this is discussed with respect to the structure of the corresponding As4S3 crystalline phases. The neutron-weighted vibrational density of states (VDOS) for vitreous Ge3As52S45, obtained with the ISIS MARI spectrometer, unambiguously confirms the existence of As4S3 molecules, as revealed by a peak at ~34meV arising from the triangle of As atoms that form the base of the molecule. MARI data recorded at ambient temperature also reveal a strong quasi-elastic component in the scattered intensity, which has been further investigated using the IN5 quasi-elastic scattering spectrometer (ILL) and indicates the presence of rotational diffusion of the As4S3 molecules; i.e. that, at ambient temperature, vitreous Ge3As52S45 behaves as a plastic glass. A possible structural model for vitreous Ge3As52S45 comprises a two-dimensional tangled GeS2 net with As4S3 molecules trapped in its folds whereas, for vitreous Ge3As52Se45, a clathrate model appears more appropriate.

Iron phosphate (Fe2O3-P2O5) glasses with a high Fe2O3 content are particularly interesting in that they exhibit short range antiferromagnetic (speromagnetic) ordering at low temperatures. Neutron diffraction techniques have been employed to investigate the atomic structure of iron phosphate glasses, as a function of composition between 30 and 44 mol% Fe2O3, and the data are compared with two structural models in which the Fe atoms are either tetrahedrally or 6-fold coordinated by oxygen. It is concluded that the structure is much more complicated than either of these models would suggest and that it includes Fe3+ ions in both tetrahedral and octahedral coordination. Fe2+ ions are also present in octahedral, and possibly 5-fold, coordination.

The first successful complete neutron diffraction anomalous dispersion experiment has been performed to investigate the role of the Sm3+ ions in the structure of vitreous Sm2O3·4P2O5. The ideal form of this technique, which employs the wavelength dependence of the real and imaginary parts of the neutron scattering length close to an absorption resonance, is used and involves measurements at two pairs of wavelengths: the real part of the scattering length is varied, keeping the imaginary part constant, and then the imaginary part is varied, keeping the real part constant. If A denotes the element with the isotope (149Sm) having the absorption resonance and X any other element present in the sample, the first measurement can be used to extract the A−A+A−X or A−A+X−X contribution to the real space correlation function, T(r), and the second yields the A−A component correlation function. For the present glass, the Sm−Sm+Sm−X contribution (X=P or O) reveals that the Sm3+ ions have an average co-ordination number, nSm(O), of 7, with a mean SmO bond length of 2.375±0.005Å, while the anomalous difference correlation function indicates that the Sm3+ ions are ∼4.6Å apart. The paper concludes with a discussion of the relative merits of the various neutron diffraction techniques for isolating individual or subsets of real-space component correlation functions.

The role of the Sm3+ ions in the structure of vitreous Sm2O3 center dot 4P(2)O(5) has been investigated using the neutron diffraction anomalous dispersion technique, which employs the wavelength dependence of the real and imaginary parts of the neutron scattering length close to an absorption resonance. The data described here represent the first successful complete neutron anomalous dispersion study on an amorphous material. This experimental methodology permits one to determine exclusively the closest Sm center dot center dot center dot Sm separation. Knowledge of the R center dot center dot center dot R (R = rare-earth) pairwise correlation is key to understanding the optical and magnetic properties of rare- earth phosphate glasses. The anomalous difference correlation function, Delta Y"(r), shows a dominant feature pertaining to a Sm center dot center dot center dot Sm separation, centred at 4.8 angstrom . The substantial width and marked asymmetry of this peak indicates that the minimum approach of Sm3+ ions could be as close as 4 angstrom. Information on other pairwise correlations is also revealed via analysis of T (r) and Delta T (r) correlation functions: Sm3+ ions display an average co-ordination number, nSm(O), of 7, with a mean Sm-O bond length of 2.375(5) A whilst the PO4 tetrahedra have a mean P - O bond length of 1.538(2) angstrom. Second- and third-neighbour correlations are also identified. These results corroborate previous findings. Such consistency lends support to the application of the anomalous dispersion technique to determine Sm center dot center dot center dot Sm separations.

The role of the Sm {sup 3+} ions in the structure of vitreous Sm {sub 2} O {sub 3}{center_dot} 4P {sub 2} O {sub 5} has been investigated using the neutron diffraction anomalous dispersion technique, which employs the wavelength dependence of the real and imaginary parts of the neutron scattering length close to an absorption resonance. The data described here represent the first successful complete neutron anomalous dispersion study on an amorphous material. This experimental methodology permits one to determine exclusively the closest Sm-Sm separation. Knowledge of the R-R (R= rare-earth) pairwise correlation is key to understanding the optical and magnetic properties of rare-earth phosphate glasses. The anomalous difference correlation function,{delta} T''(r), shows a dominant feature pertaining to a Sm-Sm separation, centred at 4.8 A. The substantial width and marked asymmetry of this peak indicates that the minimum approach of Sm {sup 3+} ions could be as close as 4 A. Information on other pairwise correlations is also revealed via analysis of T (r) and {delta} T (r) correlation functions: Sm {sup 3+} ions display an average co-ordination number, n {sub Sm (O)}, of 7, with a mean Sm-O bond length of 2.375 (5) A whilst the PO {sub 4} tetrahedra have a mean PO bond length of 1.538 (2) A. Second-and

A comprehensive neutron scattering study has been performed of hydrogenated (Si0.78H0.22), deuterated (Si0.77D0.23) and partially fluorinated deuterated (Si0.725D0.120F0.155) amorphous silicon, prepared by the glow-discharge technique. The measurements performed include diffraction, small-angle neutron scattering (SANS) and inelastic neutron scattering, and the data obtained are discussed in terms of various structural models in the literature. The real-space correlation function for Si0.77D0.23 exhibits sharp peaks at 1.49 and 2.36 angstrom , due to Si-D and Si-Si covalent bonds, respectively, while peaks centred at 3.2 and 3.8 angstrom are due to Si-D and Si-Si second-neighbour distances. High-energy resolution inelastic scattering measurements for Si0.78H0.22 show that there are approximately equal numbers of equivalent to SiH and = SiH2 groupings, there being no indication of excitations corresponding to -SiH3 groupings. The presence of molecular hydrogen is demonstrated unambiguously by the observation of the ortho-to-para conversion, via molecular rotation modes at 14.5 and 29.4 meV. The shift in the Si-H stretch modes introduced by deuteration is slightly less than the value of root 2 expected for free hydrogen, indicating a small but observable influence of the amorphous silicon matrix. The size of the cages containing the H-2 molecules has been investigated via SANS, which yields a mean Guinier radius of similar to 5-6 angstrom. In addition, the use of the H-D SANS contrast technique indicates that each cage contains on average about 60 H-2 (D-2) molecules. The data for the Si0.725D0.120F0.155 sample are consistent with a previously suggested model of network cages predominantly containing molecular SiF4.

Inelastic neutron scattering data have been measured for the vitreous forms of caesium enneaborate and pentaborate along with polycrystalline caesium enneaborate, pentaborate and triborate. The known crystal structures allow associated superstructural unit modes in the neutron weighted vibrational density of states (VDOS) of the glass forms to be identified. The widths of the features in the VDOS give additional information on the dispersive nature of the modes and hence the molecular character. To assist in the identification of larger structural units, new Raman measurements have been completed for a range of five glasses and four crystalline compositions in the same system. It is concluded that boroxol units are only formed in vitreous samples containing alkali tip to the tetraborate composition. A glass with the enneaborate composition contains both boroxol and triborate units, while glasses of pentaborate composition appeal, to contain pentaborate and possibly triborate groups.

The high temperature crystal structure of caesium enneaborate, gamma-CS2O.9B(2)O(3), has been refined using the Rietveld technique, from neutron powder diffraction data obtained with the high resolution powder diffractometer (HRPD) at the ISIS spallation pulsed neutron source. gamma-CS2O.9B(2)O(3) is tetragonal, space group P4,22, with unit cell dimensions a(0)=8.77646 +/- 0.00016 and c(0)=15.94929 +/- 0-00030 angstrom. This polymorph was formed by devitrification at high temperature, before undergoing a displacive transition to the orthorhombic polymorph, beta-CS2O.9B(2)O(3), detailed in earlier neutron studies, upon cooling below 316 K. The topology of the borate network remains unchanged, comprising two independent, interpenetrating borate networks of boroxol and triborate groups in the ratio 2:1. An analysis is presented of the detailed geometry of the borate units, and the probable existence of a third, low temperature, polymorph is discussed.

The magnitude of the network density fluctuations in vitreous silica, prepared by flame hydrolysis and from crystalline alpha-quartz, have been investigated by careful absolute measurements of the zero scattering vector limit of the structure factor, S (0). The data were obtained using the D11 instrument at the Institut Laue-Langevin, making a precise correction for multiple scattering followed by accurate normalisation to yield an absolute differential scattering cross section. If it is assumed that the density fluctuations present in the supercooled liquid at the glass transition temperature, T-g, are frozen in as the liquid cools, then S(0) may be calculated from the isothermal static compressibility, kappa(T-g), of the supercooled liquid at T, Any increase in the density fluctuations during or below the glass transition will lead to an excess contribution to S(0) above that predicted from K(T-g). Measurements of the neutron differential scattering cross section at small scattering vector magnitudes, Q, have been made in the region 0.004 <= Q <= 0.154 angstrom(-1), which extrapolate to an absolute value for S (0) of 0.0300 +/- 0.0016, in agreement with previous small angle x-ray scattering studies and with the isothermal compressibility of the liquid at the glass transition temperature, obtained from light scattering. The results are thus consistent with a random network structure incorporating 'frozen in' density fluctuations, appropriate to the liquid at the fictive temperature, and there is no evidence for the formation of crystallites on passing through the glass transition.

A neutron inelastic scattering investigation of the superstructural unit concentrations in Cs2O-B2O3 glasses has been performed. Data have also been measured for crystalline caesium borates and used to identify the associated superstructural unit breathing modes in the neutron-weighted vibrational density of states (VDOS). Effective VDOS for vitreous caesium enneaborate and crystalline caesium pentaborate, measured at < 17 K, are presented and discussed. The inelastic scattering experiments indicate that the pentaborate superstructural unit in polycrystalline caesium pentaborate is characterised by a split double peak at 95.5 and 98.6 meV, while caesium enneaborate glass has a complex signature in this region, and a peak due to the boroxol ring breathing mode at 99.9 meV.

The phenomenon of spin freezing at low temperatures in iron-containing oxide glasses resulting from antiferromagnetic interactions has been previously reported for several oxide glass systems. The temperature dependence of the DC magnetic susceptibility has been measured with a SQUID magnetometer for a series of four iron oxide–phosphorus pentoxide glasses, containing between 30 and 44mol% Fe2O3, and prepared so as to have an Fe3+ fraction of ∼0.8. Well defined cusps are observed in the susceptibilities at temperatures between 5K and 8K for the four samples, cooled in zero field and measured at 0.025T. As for classical metallic spin glass alloys, the susceptibilities measured after cooling from high temperature in this field are almost constant below the cusp temperatures, corresponding to spins freezing into a complicated configuration. Similar measurements at 0.5T also demonstrate the freezing transition but the cusps in the susceptibility after zero-field cooling are broadened. The distribution and environments of iron ions within the samples are discussed in the light of the temperature-dependent magnetic structure factors and spin correlation functions, observed in neutron diffraction experiments on the same samples, along with the nuclear real space total correlation functions.