X-ray and neutron scattering

Abstract:

We have investigated the optimum conditions for the growth of Na xCoO2 single crystals by the optical floating-zone technique. A significant reduction in Na loss, was achieved by using a high-pressure growth atmosphere. A high-pressure oxygen-rich atmosphere during crystal growth was found to reduce the presence of CO3O4 impurity phase in the grown crystal. The magnetic properties of powder and single crystal samples were measured by superconducting quantum interference device (SQUID) magnetometry and found to vary according to the preparation conditions. © 2004 Elsevier B.V. All rights reserved.

Abstract:

In this paper we report studies of the charge stripe ordering using high resolution x-ray scattering in the nickelate system La2-xSr xNiO4 with doping levels of x=0.33, x=0.30, and x=0.275. The charge stripes for all doping levels were found to be two dimensional in nature with a high degree of correlation in the a-b plane. The in-plane inverse correlation length in the lower doped systems was greater than that in the x=0.33 system and is greatest for x=0.275, consistent with the stripes becoming less correlated as the doping level is decreased from x=0.33. However, the charge ordering in the x=0.33 system was observed to be more two dimensional in nature with a greater inverse correlation length between planes. The interaction between the lattice and charge order was observed to stabilize the charge ordering, even in the absence of spin ordering. In the x=0.30 and x=0.275 systems no long-range charge order could exist without the presence of the magnetic order. In both systems the charge stripes were incommensurate and on heating the incommensurability increased towards the stable ε=0.33 value as the stripes gained thermal energy to overcome the Coulombic repulsion. In all these systems the integrated intensity of the stripe reflections showed a clear difference in behavior from previous neutron studies with an initial increase in the integrated intensity as the temperature was increased, which was not found in the neutron studies.

Abstract:

We describe a model-independent approach for the extraction of detailed lattice dynamics information from neutron powder diffraction data, based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement. Phonon dispersion curves for MgO extracted in this way are shown to reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. By means of molecular dynamics simulations, we quantify the extent to which the diffraction data are sensitive to lattice dynamics in this system.