Adrian C Barnes

Neutron and x-ray diffraction studies of liquids and glasses

The techniques of neutron diffraction and x-ray diffraction, as applied to structural studies of liquids and glasses, are reviewed. Emphasis is placed on the explanation and discussion of the experimental techniques and data analysis methods, as illustrated by the results of representative experiments. The disordered, isotropic nature of the structure of liquids and glasses leads to special considerations and certain difficulties when neutron and x-ray diffraction techniques are applied, especially when used in combination on the same system. Recent progress in experimental technique, as well as in data analysis and computer simulation, has motivated the writing of this review.

The hydration of Dy3+ and Yb3+ in aqueous solution: A neutron scattering first order difference study

The coordination of water molecules around the metal ion in acidified 1 m DyCl3, 1.0 m Dy(ClO4)3, 0.3 m Dy(ClO4)3, and 1.0 m Yb(ClO4)3 solutions in D2O have been determined by the neutron diffraction difference technique. A coordination number of eight is found for the two ions, which depends neither on the counterion, nor on the concentration. The near metal–oxygen and metal–deuterium distances are, respectively, 2.39±0.02 and 3.03±0.02 A for dysprosium and 2.33±0.02 and 2.98±0.02 A for ytterbium. A general discussion concerning the hydration of the lanthanide ions is given.

The hydration structure around chloride ions in aqueous solution

The hydration structure around chloride ions has been determined for several heavy water electrolyte solutions at various concentrations by the first-order difference method of neutron diffraction. Particular attention has been focused on aqueous zinc chloride solutions, for which results are now available. The following conclusions have been drawn from these data: (1) the chloride-water radial distribution function has a similar form in all solutions studied and is consistent with a well defined hydration geometry, with the Cl—D nearest neighbour distance, text-decoration:overlinerCID, in the range 2.2 < text-decoration:overlinerCID/A < 2.3 and a nearly linear Cl—D—O arrangement; (2) the number of water molecules in the first coordination shell of Cl–(the hydration number, text-decoration:overlinenD) is ≲6 and depends on both the ionic strength and the type of cation.

Anomalous X-ray diffraction studies of hydration effects in concentrated aqueous electrolyte solutions

Abstract The method of anomalous X-ray diffraction (AXD) was applied to two aqueous electrolyte solutions: 6 m rubidium bromide and 4 m rubidium chloride. Data from the former solution were used to determine Br− hydration and those from the latter to determine Rb+ hydration. The results are compared with those obtained from other techniques and contrasted with the hydration of other ions in their respective series. A critical appraisal of the AXD method and its potential for future studies of ionic solutions are given.

A determination of the structure of liquid using neutron diffraction and isotopic substitution

The partial structure factors and pair distribution functions for liquid were measured using the method of neutron scattering and isotopic substitution. The results show many features which are consistent with an ionic melt in which small ions move rapidly through a highly ordered liquid sub-structure in a similar way to the motion of in the fast-ion solid which exists prior to melting. The results are compared with recent molecular dynamics and ab initio molecular dynamics calculations, and the relative merits of the two simulation approaches are discussed in relation to the data.

The electronic properties of liquid Ag1-xSex

Measurements of the electrical conductivity sigma , thermoelectric power S and magnetic susceptibility of liquid Ag1-xSex alloys have been measured as a function of composition and temperature. The unusual maximum in sigma , along with the negative d sigma /dT observed around stoichiometry for Ag-S alloys is also observed in this system. The thermopower shows a p-n transition at x=0.36 with a magnitude indicating that this is a liquid semiconductor with an essentially zero energy gap. It is demonstrated that the results cannot be explained using the Kubo-Greenwood expressions for sigma and S if rigid band behaviour is assumed. The origins of this unusual behaviour are discussed in terms of an enhanced carrier mobility near the stoichiometric composition.

An anomalous x-ray diffraction study of the hydration structures of Cs+ and I− in concentrated solutions

Anomalous x-ray diffraction experiments were carried out on concentrated aqueous solutions of sodium iodide (6 molal) and cesium iodide (3 molal). Data were gathered at two energies below the absorption edges of the Cs+ and I- ions in order to avoid contributions from fluorescence. The statistics and quality of the raw data were improved by the use of a focusing analyzer crystal. Differences were taken between the data sets and used to calculate the hydration structures of Cs+ and I-. The structures found are more complex than anticipated for such large ions with relatively low charge densities and show evidence of ion-pair formation in both solutions. A two-Gaussian fit to the Cs+ data gives information about the Cs+-O and Cs+-I- correlations. The central position of the Gaussian representing the Cs+-O was fixed at 3.00 A, that is, the maximum of this contribution. The other parameters were allowed to vary freely, giving a Cs+-I- distance of 3.84+/-0.05 A and coordination numbers of 7.9 and 2.7, respectively, for the Cs+-O and Cs+-I- correlations. The results on the structure of I- in the 6 molal NaI aqueous solution were also fitted to a model based on Gaussians; this gives correlations for I- -O and I- -Na+ at 3.17+/-0.06 and 3.76+/-0.06 A with respective coordination numbers of 8.8 and 1.6. The structure of I- in the 3 molal CsI solution shows overlapping contributions due to I- -H, I- -O, and I- -Cs+. The best Gaussian fit gives two peaks centered at 3.00+/-0.08 and 3.82+/-0.04 A and shows that the latter two correlations are unresolved. The hydration structures are compared with those of other alkali and halide ions. The results are also found to be in good agreement with those obtained from standard x-ray diffraction and computer simulation.

Anomalous x-ray diffraction studies of Sr2+ hydration in aqueous solution

Anomalous x-ray diffraction experiments were carried out at two energies below the K edge of strontium on a 3.5 molal solution of strontium chloride in water. The first order difference method was applied to the corrected and normalized data sets, and a difference function for the Sr2+ coordination obtained. Fourier transformation of this function gave a modified total radial distribution function for Sr2+, which when analyzed showed that there are eight to nine nearest neighbor water molecules sited at 2.67 A from the Sr2+ ion. A second coordination shell is also apparent in the range 4.5⩽r(A)⩽5.5 which can accommodate Cl− counterions and a second shell of water molecules. The results obtained for the distance between Sr2+ and O are in excellent agreement with those obtained from standard x-ray diffraction, extended x-ray absorption fine structure, and neutron diffraction isotopic substitution. The hydration structure obtained for Sr2+ is compared with the structure of two other ions in the alkaline eart...

The electrical conductivity and thermopower of liquid Ag1-xSx alloys near the stoichiometric composition Ag2S

The electronic properties of liquid Ag1-xSx have been measured as a function of temperature and concentration near the stoichiometric composition Ag2S. The unusual negative temperature dependence of the conductivity at stoichiometry found previously has been confirmed, and shown to extend over the composition range 0.3250<or=x<or=0.3345. The concentration dependence of the conductivity shows a sharp peak at the stoichiometric composition reaching a value of 250 Omega -1 cm-1 at 1000 degrees C; such a feature has never been observed in a liquid semiconductors with a comparable conductivity. The thermopower of liquid Ag1-xSx is negative and shows a broad minimum centred around the composition x=0.315.

Structure of molten TbCl3 measured by neutron diffraction

The total structure factor of molten TbCl3 at 617oC was measured by using neutron diffraction. The data are in agreement with results from previous experimental work but the use of a diffractometer having an extended reciprocal-space measurement window leads to improved resolution in real space. Significant discrepancies with the results obtained from recent molecular dynamics simulations carried out using a polarizable ion model, in which the interaction potentials were optimized to enhance agreement with previous diffraction data, are thereby highlighted. It is hence shown that there is considerable scope for the development of this model for TbCl3 and for other trivalent metal halide systems spanning a wide range of ion size ratios.