A neutron diffraction investigation of the structure of vitreous zinc chloride

A survey is presented of the AX₂ glasses and the corresponding crystalline polymorphs. The relationship between ZnCl₂ 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 AX₂ glasses is likely to be small. A neutron diffraction investigation of vitreous ZnCl₂ using the D4 diffractometer at ILL Grenoble shows that the structure of vitreous ZnCl₂ comprises a distorted random close packed array of Cl^? ions with the Zn²⁺ ions in tetrahedral holes, arranged in such a way as to maximise corner sharing of the resulting ZnCl₄^2? 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 ZnCl₂ glass is δ-ZnCl₂ and not the α-form as previously reported.     

A neutron diffraction and MAS-NMR study of the structure of fast neutron irradiated vitreous silica

Neutron diffraction and magic angle spinning (MAS)-NMR measurements have been performed on vitreous silica before and after irradiation to a dose of 2.8×10²⁰ fast neutrons cm⁻². On fast neutron irradiation the first peak in the diffraction pattern is reduced in height, broadened and shifted to a slightly higher scattering vector, reflecting the increase in density of 2.4%. This may be interpreted as an enhancement of smaller scale structures, consistent with irradiation generating a higher proportion of three-membered rings as indicated by Raman data. In real space, there is a corresponding small increase in the Si---O bond length which is consistent with a reduction in the mean Si---O---Si bond angle of ≈9.5° as indicated by the MAS-NMR data. Fast neutron irradiation also leads to a peak in the difference correlation function at ≈3.2Å which has previously been ascribed to oxygen atom interstitials, but may be identified as the Si---(2)O distance within a three-membered ring.     

The structure of amorphous carbon tetrachloride by neutron diffraction

A vapour-deposited sample of amorphous CCl₄ at 10 K has been studied by neutron diffraction. The measured structure factor confirms that the basic molecular unit is retained in the amorphous phase and a form-factor fit to the high Q-value data gives a CCl bond length of (1.770±0.002) Å. The inter-molecular function, D_M(Q) shows more structure than for the liquid phase and transformation to a real-space distribution function confirms that this is due to a much stronger orientational correlation between adjacent molecules in the amorphous solid. The density is found to be (25±5)% higher than in the liquid and the structural properties can be well described using a reference interaction site model (RISM).     

Network structure of multi-component sodium borosilicate glasses by neutron diffraction

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65 − x)SiO_2. · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5–15 mol%. The maximum momentum transfer of the experimental structure factor was 30 Å⁻¹, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO₄ units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40 Å and 1.60 Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO₃ units, and the silicon rich network is formed by a mixed continuous network of ^[4]Si–O–Si^[4] with several different ^[4]B–O–Si^[4] and ^[3]B–O–Si^[4] linkages.     

Refinement of the crystal and molecular structure of tetraacetylethane by neutron diffraction

The structure of tetraacetylethane has been determined by neutron diffraction. The compound exists in the dienolic form, the enolic hydrogen being intramolecularly bonded to a neighbouring oxygen in a clearly asymmetric mode. The acetylacetonate residues are essentially planar.     

Study by x-ray and neutron diffraction of the structure of ex-carbonyl amorphous iron

Ex-carbonyl amorphous iron is a heterogeneous finely powdered material. Its texture established by standard physico-chemical analysis is compatible with the diffraction data: two phases have been identified, an amorphous ferromagnetic phase homogeneous up to 300 Å containing less than 1% carbon in solution and an unstable phase coming from a part of the CO from the decomposition of carbonyl and representing 7.2% by weight. The pair distances from both X-ray and neutron diffraction spectra are identical after taking into account measurement error: the shorter pair distances are those already found in icosahedral structures, the higher distances, identified up to 19 Å^?1, are simple exact combinations of the shorter ones.     

Liquid structure of Rb–Hg alloys studied by neutron diffraction

The structures of liquid Rb–Hg alloys were studied as a function of composition by neutron diffraction. In the intermediate Rb concentration range, the obtained structure factors show a small prepeak, which may be an evidence of the formation of Hg polyanion units in liquids. The Reverse Monte Carlo (RMC) analysis was applied to separate the total radial distribution function into the corresponding partial radial distribution functions. Up to 10 at.% Rb, no obvious changes are found for the first peak position of the partial radial distribution functions of the Hg–Hg pair and that of the Hg–Rb pair. The first peak position between the Hg–Rb pairs increases above 20 at.% Rb. In addition to the first peak, a subpeak between Hg–Hg pairs can be seen in the large distance. At 60 at.% Rb, the nearest neighbor distance between Hg atoms shows the closest value in the concentration range studied. These results indicate that with the progress of charge transfer the solvation structure in the dilute Rb concentration range changes into the structure containing polyanions composed of Hg species.     

Crystal structure of a salt of the pyridinium-pyridine ion by X-ray and neutron diffraction

The crystal structure of [(C₅H₅N)₂H]⁺[ZnBr_3/2Cl_3/2(C₅H₅N)]^– has been determined and refined by full-matrix least-squares methods using X-ray (R, on ¦F _o¦, = 0.045 with 2291 reflections) and neutron (¯R, on ¦F _o¦, = 0.035 with 2030 reflections) diffraction data. The crystals of the compound are monoclinic, space groupP2₁/c witha = 12.728(1),b = 11.600(1),c = 13.354(1) Å, = 107.38(1) °,Z = 4,D _x = 1.68, andD _m(flotation) = 1.69 g cm^–3. The X-ray and neutron intensities were measured by the counter -2 scan technique with CuK X-rays ( = 1.5418 Å) and neutrons with = 1.109 Å, respectively. An intramolecular N--H hydrogen bond holds pairs of pyridine molecules together in the lattice [N N, 2.737(3) Å]. The hydrogen atom is very asymmetrically disposed with N-H distances of 1.086(7) Å and 1.658(6) Å. The coordination about the zinc atom is tetrahedral with the nitrogen atom of the pyridine molecule in one tetrahedral position and different mixtures of the two halogen atoms in the other three positions. The important average bond distances from the neutron study are Zn-N [2.073(3) Å], Zn-Cl [2.251(2) Å], Zn-Br [2.394(3) Å], N-C [1.334(5) Å], C-C [1.378(5) Å], and C-H [1.075(7) Å].     

Investigation of the structure of C60 fullerene halides by the neutron and synchrotron diffraction methods

The crystal and molecular structures of bromofullerenes and chlorofullerenes, namely, C₆₀Br₆ · 0.5PhCl · 0.5Br₂, C₆₀Br₈ · TiBr₄ · Br₂, C₆₀Br₂₄ · Br₂, and C₆₀Cl₂₄ · TiCl₄, are determined and refined using neutron and synchrotron diffraction. The interatomic distances are determined with an accuracy higher than the accuracy in the determination of the corresponding parameters for the same molecular structures with the use of X-ray laboratory equipment.     

Crystal structure of non-stoichiometric copper selenides studied by neutron scattering and X-ray diffraction

Structural characteristics of non-stoichiometric copper selenides were studied by the elastic neutron and X-ray scattering techniques. Rietveld analysis was used to refine the structure of the high-temperature β-phase of the Cu_1.75Se, Cu_1.78Se, and Cu_1.83Se samples. The homogeneity ranges of the cubic phase were determined. The modification of the crystal structure accompanying the β-α phase transition was studied for Cu_1.75Se and Cu_1.98Se compounds within the 443-10 K temperature range. It was shown that the phase transition is accompanied by distortions of the fcc lattice and the ordering of copper ions.     

On the correlation between void-species structure of vitreous arsenic selenide studied with X-ray diffraction and positron annihilation techniques

The structure of nanoscale void-species has been studied in glassy arsenic selenide g-As₂Se₃ by combining the first sharp diffraction peak in X-ray diffraction (FSDP-related XRD), treated within Elliott’s void-based model, and positron annihilation lifetime spectroscopy (PALS) within two-state positron trapping model. By suggesting that the same nanovoids are responsible for both FSDP and PALS data, the analytical correlation relationship between the FSDP position, Q₁, and nanovoid diameter, D, has been presented in the form of Q₁ = 2.3 · π/D.     

Structure of chalcogenide glasses by neutron diffraction

The purpose of this work is to study the change in the structure of the Ge–Se network upon doping with Ag. We report here a neutron diffraction study on two glasses of the system Ag_x(Ge_0.25Se_0.75)_100−x with different silver contents (x = 15 and 25 at.%) and for two different temperatures (10 and 300 K). The total structure factor S(Q) for the two samples has been measured by neutron diffraction using the two-axis diffractometer dedicated to structural studies of amorphous materials, D4, at the Institut Laue Langevin. We have derived the corresponding radial distribution functions for each sample and each temperature, which gives us an insight about the composition and temperature dependence of the correlation distances and coordination numbers in the short-range. Our results are compatible with the presence of both GeSe_4/2 tetrahedra and Se–Se bonds. The Ag atoms are linked to Se in a triangular environment. Numerical simulations allowing the identification of the main peaks in the total pair correlation functions have complemented the neutron diffraction measurements.     

Investigation of the lithium-nitrogen melt microstructure by neutron diffraction

The microstructure of Li-N melts with nitrogen impurity contents of 1.3 and 3.5 at % has been investigated by neutron diffraction. The main microstructural characteristic of the material (total structure factor) and the partial structure factors of the melt components and the radial distribution functions of particles have been obtained. It is established that there is no prepeak in the structure factor of Li-N melts, which indicates the existence of clusters of a certain size in the melt. Analysis of the partial structural characteristics suggests the existence of nitrogen impurity in Li-N melts in the form of lithium nitride Li₃N.     

X-ray spectroscopic investigation of the structure of silica,silicates and oxides in the crystalline and vitreous state

The Si Kα_1,2 emission lines and their satellite lines α′, α₃ and α₄ were measured for several samples of vitreous silica (Suprasil W, Infrasil, Suprasil), sodium silicate glasses (8, 15, 20 and 25 wt% Na₂O), and crystalline Mg₂SiO₄ (Forsterite). The observed shifts of the peak positions indicate a systematic increase of the electron density on the silicon atoms with increasing break-up of the SiO₂ network by OH^? or alkali ions. These results are compared with information from the corresponding Si Kβ and O K emission bands and also with the O K emission bands from quartz, MgO and Al₂O₃. They are discussed on the basis of the MO theory and are compared with the characteristic physical properties and structure of silica and silicate glasses. Both the O K and Si Kβ emission bands are closely related to the electronegativities of the relevant metal atoms of the oxides and glasses.     

Neutron diffraction investigation of the hydrogen positions in the crystal structure of monetite,CaHPO4

An occupation-factor analysis using single-crystal neutron diffraction data has shown that the hydrogen atoms in the structure of the mineral monetite, CaHPO₄, occupy hydrogen-bonding sites closely similar to those predicted in a previous X-ray refinement. Least-squares refinement in space groupP¯1 of the potential coordinates and isotropic temperature parameters for the three distinct proton sites led to a resultantR of 0·10 over the 371 independent neutron structure factors. Detailed analysis showed that the distribution of the hydrogen atoms was almost certainly ordered in symmetryP1 with only half of the O(6) atoms (in space groupP¯1) carrying hydrogen atoms; the short O(7)-P(1)... O(7) hydrogen bond of 2·44 Å across what would be a symmetry centre inP¯1 is probably unsymmetrical. Minute piezo- and pyroelectric effects confirmedP1 as the space group. Further refinement of the X-ray data in space groupP1 indicated that the deviations from centrosymmetry must be small and, so far as can be detected, confined to the hydrogen atoms.We thank Mr James R. Lehr for the crystal, Prof. W. G. Perdok for the piezoelectric measurement, and Dr. G. Ferraris for a useful discussion. We are also grateful to Mr. D. H. C. Harris and the staff of the University Support Group for help at AERE, and to the S.R.C. for providing access to neutron and computational facilities and for financial support to WAD.     

The structure analysis liquid Sb2Se3 by neutron diffraction

The radial distribution analysis of liquid Sb₂Se₃ at 670, 720 and 770°C is carried out by the neutron diffraction method. A small but apparent maximum is found at $\text{K = 1.2}\text{A}\text{?}{}^{\mathrm{?1}}$ in the structure factor. The interatomic distance and the coordination number are 2.74 Å and 2.84 respectively, at 670°C. These results suggest that intramolecular bonds in the crystalline Sb₂Se₃ remain unbroken in the liquid state. The general feature of the structure factor and the distribution function for liquid Sb₂Se₃ is similar to that for the amorphous state. However, it is clear that the liquid phase tends to have a similar atomic arrangement to that of liquid tellurium.     

The interstitial structure of vitreous silica

A careful analysis of gas solubility data provides a unique tool for characterizing glass network geometry. The density of interstitial (solubility) sites in vitreous silica and its crystalline analog, high cristobalite, was reviewed. Close inspection of high cristobalite indicates that there are eight solubility sites per unit cell giving a bulk density of 2.34 × 10²² sites/cm³. A statistical thermodynamic analysis of experimental solubility data for helium and neon in high cristobalite correlates with this calculated value. Correcting this value for the slightly lower bulk density of vitreous silica gives an estimated density of solubility sites in vitreous silica of 2.22 × 10²² sites/cm³. Literature data for solubility site density (obtained from the same statistical thermodynamic analysis) depend strongly on the dissolved gas species. This implies a statistical distribution of effective site diameters. The probability distribution function is adequately represented as log-normal.     

X-ray and neutron diffraction study of the defect crystal structure of the as-grown nonstoichiometric phase Y0.715Ca0.285F2.715

This work begins a series of papers aimed at studying the defect structure of nonstoichiometric phases R _{1 ? y} Ca _y F_{3 ? y} with a tysonite-type (LaF₃) structure. In the single-crystal structure of Y_0.715Ca_0.285F_2.715 with a tysonite-type small unit cell (sp. gr. P6₃/mmc, a = 3.9095(2) Å, c = 6.9829(2) Å; Z = 2; R _w = 2.16%), the displacements of Y³⁺ cations and F^2? anions from 6₃ symmetry axes were observed for the first time. The X-ray diffraction pattern shows weak satellites insufficient for structural calculations. The LaF₃ structure type is stabilized up and down on the temperature scale due to anion vacancies and the symmetrizing effect of Ca²⁺ cations lying on 6₃ symmetry axes. At 120°C the fluoride-ion conductivity in the nonstoichiometric phase Y_0.715Ca_0.285F_2.715 is five orders of magnitude higher than that in the stoichiometric phase β-YF₃. The transition to a superionic state is caused by a deviation from stoichiometry and is not associated with reconstructive phase transformation.     

Investigation of the structure and properties of quartz in the α-β transition range by neutron diffraction and mechanical spectroscopy

The paper reports on the results of complex investigations into the physical properties of synthetic quartz single crystals and quartz powders in the temperature range of the α-β transition with the use of neutron diffraction and mechanical spectroscopy. The crystal structure of quartz powders with different average sizes of grains is determined in the temperature range up to 620°C and in the α-β transition temperature range. The temperature dependences of the internal friction and the resonant frequency for quartz samples in the vicinity of the phase transition temperature are obtained upon excitation of vibrations in the planes parallel and perpendicular to the Z axis of the quartz crystal. The temperatures at the maxima of the internal friction in the range 560–620°C are determined. The assumptions regarding the possible reasons for the shift of the phase transition temperature are made. It is revealed that the internal friction is characterized by a maximum that is observed in the vicinity of 350°C and is not related to the structural transformations in quartz.