Series Of Neutron Diffraction Experiments Research Articles

Proton Dynamics in Palladium-Silver: An Inelastic Neutron Scattering Investigation.

Proton dynamics in Pd77Ag23 membranes is investigated by means of various neutron spectroscopic techniques, namely Quasi Elastic Neutron Scattering, Incoherent Inelastic Neutron Scattering, Neutron Transmission, and Deep Inelastic Neutron Scattering. Measurements carried out at the ISIS spallation neutron source using OSIRIS, MARI and VESUVIO spectrometers were performed at pressures of 1, 2, and 4 bar, and temperatures in the 330–673 K range. The energy interval spanned by the different instruments provides information on the proton dynamics in a time scale ranging from about 102 to 10−4 ps. The main finding is that the macroscopic diffusion process is determined by microscopic jump diffusion. In addition, the vibrational density of states of the H atoms in the metal lattice has been determined for a number of H concentrations and temperatures. These measurements follow a series of neutron diffraction experiments performed on the same sample and thus provide a complementary information for a thorough description of structural and dynamical properties of H-loaded Pd-Ag membranes.

Neutron Diffraction Studies of the Molecular Compound [Co2(bta)]n (H4bta =1,2,4,5-Benzenetetracarboxylic Acid): In the Quest of Canted Ferromagnetism

The exchange mechanism and magnetic structure of the organic-inorganic layered molecule-based magnet [Co2(bta)]n (1) (H4bta =1,2,4,5-benzenetetracarboxylic acid) have been investigated through variable-temperature magnetic susceptibility measurements and supported with a series of neutron diffraction experiments. Cryomagnetic studies have shown an antiferromagnetic ordering at a transition temperature of 16 K that is followed by the appearance of a weak ferromagnetism below 11 K. The weak antiferromagnetic interlayer interaction plays an important role in this system in spite of the long interlayer separation. A ferromagnetic ordering is induced by applied magnetic fields greater than 1800 G (metamagnetic behavior), and a slow magnetic relaxation from this ferromagnetic phase to the antiferromagnetic one is observed. The magnetic structure of 1 has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and was found to be of antiferromagnetic nature with the local cobalt(II) spins (magnetic moments) being aligned ferromagnetically in the ac plane and antiferromagnetically coupled along the crystallographic b axis. No evidence for a long-range spontaneous ferromagnetic component below 11 K was observed in the neutron experiment.

Methanol-water solutions: a bi-percolating liquid mixture.

An extensive series of neutron diffraction experiments and molecular dynamics simulations has shown that mixtures of methanol and water exhibit extended structures in solution despite the components being fully miscible in all proportions. Of particular interest is a concentration region (methanol mole fraction between 0.27 and 0.54) where both methanol and water appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extremal values. The observed concentration dependence of several of these material properties of the solution may therefore have a structural origin.

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi2B2C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T N, but no changes are seen at T F.

Structural and magnetic phase diagram of the series (Tb xY 1− x)Cu

In order to account for the structural and magnetic properties in the RCu intermetallic compounds, we have undertaken a series of neutron diffraction experiments in (Tb x Y 1− x )Cu ( x = 0, 0.2, 0.4, 0.6, 1) in the temperature range 2.5 to 300 K. The powerful neutron technique was used to obtain the temperature for the para-antiferromagnetic transition and the structural transformation which has allowed to delimit the different regions in the magnetic and structural phase diagram.

The structural properties of liquid, solid and amorphous sulphur

A series of neutron diffraction experiments have been used to investigate the properties of liquid, solid and amorphous sulphur. The liquid was studied from 131 degrees C to 230 degrees C, i.e. at conditions around the λ -transition (Tλ=159 degrees C). The amorphous sample was studied at -196 degrees C and the crystalline solid at room temperature. The data have been transformed to give an accurate pair correlation function g(r) from which the absolute numbers of atoms in given molecular configurations may be determined for distances less than 8 AA. A comparison of the solid and liquid g(r) data indicates which intermolecular bonds are broken on melting. The liquid data seem to be inconsistent with the picture of 'crown'-shaped S8 molecules below Tλ , and with the proposed S8-to-polymer transition at Tλ. Also it has been found that the diffraction pattern may be analyzed as though the liquid was an assembly of roughly spherical molecular units. In this case each unit would contain about six atoms, and the λ-transition could be related to the percolation limit for these units. The amorphous material was studied at the quenching temperature of -196 degrees C and its structure is intermediate between that of the parent liquid and the crystalline solid. It is concluded that a fundamental diffraction analysis of the states of sulphur has thrown new light on long standing problems.

The structure of aqueous solutions

A series of neutron diffraction experiments has been carried out on solutions of NiCl 2, NaCl and BaCl 2 in heavy water. Both the concentration of the solute and the degree of isotopic enrichment were varied in order to investigate whether the multiple-pattern method, which has been used previously to determine the partial structure factors for simple liquids, can be applied to aqueous solutions. It is concluded that the multiple-pattern method is feasible. Some general comments on the structural information contained in the single-pattern data are made.