X-ray and neutron scattering

Abstract:

We use neutron total scattering measurements with reverse Monte Carlo analysis methods incorporating an atom-swapping algorithm to identify the short-range Ca/Sr cation ordering within the Ca(x)Sr(1-x)TiO(3) solid solution (compositions x = 0.2,0.5,0.8). Our results show that nearest-neighbour pairs have a strong tendency for unlike Ca/Sr first-neighbour coordination in the x = 0.2 and 0.5 cases. In the x = 0.5 case the Ca/Sr ordering results in a structure with space group P 2(1)nm. In contrast, there is much less short-range cation ordering in the x = 0.8 case.

Abstract:

A new approach to the reverse Monte Carlo analysis of total scattering data from polycrystalline materials is presented. The essential new feature is the incorporation of an explicit analysis of the Bragg peaks using a profile refinement, taking account of the instrument resolution function. Other new features including fitting data from magnetic materials, modelling lattice site disorder and new restraint and constraint options. The new method is demonstrated by a brief review of studies carried out during its development. The new program RMCProfile represents a significant advance in the analysis of polycrystalline total scattering data, especially where the local structure is to be explored within the true constraints of the long-range average structure.

Abstract:

This Special Issue contains a collection of papers reflecting the content of the third workshop on reverse Monte Carlo (RMC) methods, held in a hotel on hills overlooking Budapest at the end of September 2006. Over forty participants gathered to hear talks and discuss a broad range of science based on the RMC technique in very convivial surroundings. Reverse Monte Carlo modelling is a method for producing three-dimensional disordered structural models in quantitative agreement with experimental data. The method was developed in the late 1980s and has since achieved wide acceptance within the scientific community [1]. It is particularly suitable for studies of the structures of liquid and amorphous materials, although it may also be applied effectively to the structural analysis of disordered crystalline systems. Since the previous RMC workshop in 2003 [2] there have been several developments in the technique, particularly as applied to crystals, and in the range of its application, most noticeable being the routine modelling of multiple data sets for a given problem; the latter growing through the increasing quality and availability of x-ray total scattering data from synchrotron x-ray sources. The RMC workshop was particularly beneficial, providing a forum for those workers in the field to take stock of past achievements and to look forward to future developments. It is our hope that the collection of papers within this Special Issue will also communicate this to the wider scientific community, providing a balance between papers that have more of an introductory review flavour and those that concentrate on current state of the art research opportunities using the RMC method. Furthermore, by including a small number of papers from colleagues working on similar disordered problems with complementary analysis techniques, we hope that the RMC method may be placed in a broader scientific context. The papers within this special issue have been arranged into four groups: those concerning liquids (1-8), amorphous (9-13) and crystalline materials (14-17) and those of a more general nature (18-23). Within these groupings, there are descriptions of RMCProfile (18) and RMCt (23), programs which use RMC methods to analyse total scattering from crystalline materials and to model inelastic neutron scattering data, respectively. There is also work using the related EPSR (6) and PDFfit (19) techniques, developments of the RMC method for analysis of single crystal electron diffraction (16) or polarised neutron diffraction (7), and examples of simultaneous RMC modelling of neutron and x-ray total scattering and XAS data (13, 10) . We are very grateful to IoP Publishing for their willingness to publish the proceedings of this meeting in a Special Issue of Journal of Physics: Condensed Matter. References [1] McGreevy R L 2001 J. Phys.: Cond. Matter 13 R877 [2] RMC-2 Workshop Proceedings 2005 J. Phys.: Cond. Matter 17 S1-S174.

Abstract:

We investigate the spin state of LaCoO3 using state-of-the-art photoemission spectroscopy and ab initio band structure calculations. The GGA+U calculations provide a good description of the ground state for the experimentally estimated value of electron correlation strength, U. In addition to the correlation effect, spin-orbit interaction is observed to play a significant role in the case of intermediate spin and high spin configurations. The comparison of the calculated Co 3d and O 2p partial density of states with the experimental valence band spectra indicates that at room temperature, Co has dominant intermediate spin state configuration and that the high spin configuration may not be significant at this temperature. The lineshape of the La 5p and O 2s core level spectra could be reproduced well within these ab initio calculations.

Abstract:

ZrW2O8 undergoes a high-pressure amorphization transition above 1.5 GPa to a phase which is recoverable to ambient conditions. Reverse Monte Carlo modeling of neutron and x-ray total scattering data from ZrW2O8 recovered from approximately 4 GPa shows that the large increase in density on pressurizing ZrW2O8 is accommodated within the structure by increased bonding between the WO4 tetrahedra. This increases the tungsten coordination; changes to the ZrO6 octahedral environment are not required. This densified crystal-based model, which contains significant local disorder within a distorted periodic structure, is also in reasonable agreement with x-ray and neutron total scattering data measured in situ at high pressure.