Neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations have been used to examine the structuring of solvent around D-glucose in aqueous solution. As expected, no significant tendency for glucose molecules to aggregate was found in either the experiments or the simulation. To the extent that solute pairing does occur as the result of the high concentration, it was found to take place through hydroxyl-hydroxyl hydrogen bonds, in competition with water molecules for the same hydrogen-bonding sites. A detailed analysis of the hydrogen-bonding patterns occurring in the simulations found that the sugar hydroxyl groups are more efficient hydrogen bond donors than acceptors. From the comparison of the MD and NDIS data, it was found that while the modeling generally does a satisfactory job in reproducing the experimental data the force fields may produce sugar rings that are too rigid and thus may require future revisions.

The combination of neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations to characterize the structuring in an aqueous solution of the denaturant guanidinium chloride is described. The simulations and experiments were carried out at a concentration of 3 m at room temperature, allowing for an examination of any propensity for ion association in a realistic solution environment. The simulations satisfactorily reproduced the principal features of the neutron scattering and indicate a bimodal hydration of the guanidinium ions, with the N-H groups making well-ordered hydrogen bonds in the molecular plane, but with the planar faces relatively deficient in interactions with water. The most striking feature of these solutions is the rich ion-ion ordering observed around the guanidinium ion in the simulations. The marked tendency of the guanidinium ions to stack parallel to their water-deficient surfaces indicates that the efficiency of this ion as a denaturant is due to its ability to simultaneously interact favorably with both water and hydrophobic side chains of proteins.

Neutron diffraction experiments were carried out at ambient temperature on aqueous solutions of glucose at concentrations of 1, 3, and 5 molal. The difference methods of neutron diffraction and isotopic substitution were used to determine various structure factors associated with all the exchangeable hydrogen atoms of the solutions. The results imply that the hydration of the glucose molecule is not concentration sensitive. It is also found that the average water structure itself is relatively unperturbed by the presence of glucose.

The principles for simultaneous measurements of the electrical conductivity and the permeability of materials ranging from ionic to semiconductors to insulators, using a non-invasive, electrodeless method, are presented succinctly. Recent results obtained for a variety of materials at room temperature and at the melting point illustrate the validity of the method. Special emphasis will be given to the fact that transport properties are now accessible over a regime of temperature never achieved before. Of particular interest is the fact it is now possible to monitor the inductance as a function of cooling, allowing a determination of changes accompanying the electrical properties and the magnetisation in the liquid and supercooled states.

Measurements of the thermophysical and structural properties of liquid materials at high temperature have undergone considerable development in the past few years. Following improvements in electromagnetic levitation, aerodynamic levitation associated with laser heating has shown promise for assessing properties of different molten materials (metals, oxides, and semiconductors), preserving sample purity over a wide range of temperatures and under different gas environments. The density, surface tension and viscosity are measured with a high-speed video camera and an image analysis system. Results on nickel and alumina show that small droplets can be considered in the first approximation to be under microgravity conditions. Using a non-invasive contactless technique recently developed to measure electrical conductivity, results have been extended to variety of materials ranging from liquid metals and liquid semiconductors to ionically conducting materials. The advantage of this technique is the feasibility of monitoring changes in transport occurring during phase transitions and in deeply undercooled states.

The electrical conductivity of levitated liquid elemental boron was measured near the melting point using a contactless electrical conductivity technique. A phase change is clearly detected in the course of laser heating of a 2 mm diameter boron sphere levitated aerodynamically. The value obtained for the electrical conductivity sets liquid boron among the liquid semiconductors and establishes that the semiconducting behavior survives the melting process contradicting an earlier report that a semiconductor-to-metal transition occurs.

In high temperature liquid alloy semiconductors, the electronic conductivity ( ) is usually in the range 5 - 500 -1 cm-1 . Several workers have long argued that, since such values are lower than those predicted by the `Mooij' limit or the Ioffe-Regel criterion (kF l > 1), the carriers in such systems are either localized or on the threshold of localization, i.e. they are characterized by a low mobility. There are no direct ways to measure the mobility of current carriers in high temperature liquids but we shall show that, with reasonable assumptions, reliable estimates of the mobility can be made by combining electron transport and magnetic susceptibility data. Our conclusions will challenge the idea that the apparent metal to non-metal transition observed in liquid alloy semiconductors is related to disorder induced localization.

The electrical conductivity, , and thermopower, S , of molten (AgX)1-c (Ag2 Se)c mixtures (X = Br, I) have been measured as a function of temperature. The conductivity of the mixture is observed to decrease rapidly on the addition of AgX to Ag2 Se and reaches a minimum on the AgX side. The thermopower on the Ag2 Se side decreases gradually as the concentration of AgX increases until c= ~0.4 at which point there is rapid change to large positive thermopowers. The value of S (+325 µV K-1(AgBr), +420 µV K-1(AgI)) on the AgX rich side decreases rapidly on addition of Ag2 Se with the result that a minimum value of S (~-125 µV K-1 ) is observed at the composition c= ~0.4 in both mixtures. A discussion of these results is presented in terms of the change from predominantly electronic transport in liquid Ag2 Se to the predominantly ionic transport in AgX. The increasing importance of the contribution of the inhomogeneous thermopower to the total thermopower of the salt rich compositions is also discussed. The decrease in the electronic conductivity with composition for c 0.6 is analysed in terms of the Kubo-Greenwood equations.

The electrical conductivities of aerodynamically levitated liquid spheres have been determined by an electrodeless method. It is shown that this technique is reliable over a wide range of temperatures; results are presented for a variety of systems including metals, semiconductors at room temperature and at their melting points, and solid and liquid Al2O3.

GEM, the General Materials Diffractometer, will replace LAD on beamline S7 at ISIS, the UK pulsed neutron source. GEM has been designed to be both a good-resolution, high-flux powder diffractometer for crystalline samples, and a wide-angular-range, high-flux, low-background instrument for disordered materials studies. The combination of high count-rate and good resolution will advance areas such as time-resolved and small sample diffraction. Further, we anticipate the use of increasingly complex sample environment and the exploitation of the high flux of longer wavelength neutrons from the cold methane moderator particularly for magnetic powder diffraction.

Several materials have been identified over the past few years as promising candidates for the development of new generations of magnetoresistive devices. These range from artificially engineered magnetic multilayers1 and granular alloys2,3, in which the magnetic-field response of interfacial spins modulates electron transport to give rise to ‘giant’ magnetoresistance4, to the manganite perovskites5,6,7, in which metal–insulator transitions driven by a magnetic field give rise to a ‘colossal’ magnetoresistive response (albeit at very high fields). Here we describe a hitherto unexplored class of magnetoresistive compounds, the silver chalcogenides. At high temperatures, the compounds Ag2S, Ag2Se and Ag2Te are superionic conductors; below ∼400 K, ion migration is effectively frozen and the compounds are non-magnetic semiconductors8,9 that exhibit no appreciable magnetoresistance10. We show that slightly altering the stoichiometry can lead to a marked increase in the magnetic response. At room temperature and in a magnetic field of ∼55 kOe, Ag2+δSe and Ag2+δTe show resistance increases of up to 200%, which are comparable with the colossal-magnetoresistance materials. Moreover, the resistance of our most responsive samples exhibits an unusual linear dependence on magnetic field, indicating both a potentially useful response down to fields of practical importance and a peculiarly long length scale associated with the underlying mechanism.

The electrical conductivity , thermopower S and magnetic susceptibility of liquid have been measured as a function of c. It is found that decreases monotonically on addition of AgCl to S and reaches a value typical of a molten salt when at which point a semiconductor to molten salt transition takes place. The thermopower is negative for c > 0.3 at which composition it changes sign. The magnetic susceptibility measurements show a steady increase with c, consistent with the changes in the ionic diamagnetism and a significant Pauli contribution from the electrons for the S rich compositions. The behaviour of the system is analysed and described in terms of a simple ionic band picture using a Kubo - Greenwood formalism in which the conductivity gap increases with increasing concentration of AgCl.

Measurements of the electrical conductivity and thermopower of liquid Ag1 − cSec alloys have been extended to the composition range c ≥ 0.4 by using an argon over pressure of 10 bar to prevent vaporisation of the Se. The composition dependence of the conductivity exhibits a minimum and a broad maximum near the compositions c = 0.4 and 0.65, respectively. The corresponding thermopower data show a p-n transition at c = 0.36. The data have been analysed using equations derived from the Kubo-Greenwood expressions.

The electrical conductivity $\ensuremath{\sigma}$ and thermoelectric power $S$ data have been measured in two semiconductors that undergo a solid-state transformation to a dynamically disordered phase about 100 \ifmmode^\circ\else\textdegree\fi{}C below the melting point. At the transition, $\ensuremath{\sigma}$ for NaSn is reduced while that for CsPb exhibits a dramatic rise. For both semiconductors, $S$ is reduced in magnitude and changes sign at the transition. These results are consistent with a model which explicitly allows for the interactions between mobile ions and electrons and which provides a unified explanation of the transport properties of these compounds and other fast ion conductors with appreciable electronic conductivity.

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 dsigma/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.

The low-temperature behavior of transport properties across an Anderson-type metal-insulator transition is considered. It is shown that previous discussions are flawed; in particular, we demonstrate that the thermopower S is not divergent as the mobility edge E(c) is approached from the metallic side. At E(c), S achieves a temperature-independent value, determined solely by the conductivity index nu. Furthermore, far from being universal in the metallic phase, the Lorentz number changes as the mobility edge is approached and, in addition, depends on nu. We also prove that, contrary to previous assertions, the thermal conductivity varies like T(v+1) as T-->0.

Neutron diffraction from molten K{sub x}Te{sub 1{minus}x} is reported for x = 0.12 and x = 0.50 semiconducting alloys. The measured radial distribution functions (rdf) demonstrate the persistence of covalently bonded tellurium in the liquid. The rdf of the liquid K{sub 0.12}Te{sub 0.88}, which is dominated by the Te-Te contribution, is remarkably similar to that of pure liquid tellurium, the notable exception being that the nearest neighbor peak is largely resolved and found to have a coordination number slightly less than 2. The K{sub 0.50}Te{sub 0.50} rdf clearly indicates Te-Te pairing in the melt, and unexpected departure from the presumed similar Cu{sub 0.50}Te{sub 0.50}. These paired tellurium are most likely of the form (Te{sub 2}){sup 2{minus}} Zintl ions.