Interatomic potential parameters for potassium tetrachlorozincate and their application to modelling its phase transformations

An empirical fitting procedure is applied to derive interatomic potential parameters for a model phase transition system, namely potassium tetrachlorozincate (K(2)ZnCl(4)). The derived potential is found to reliably model the known crystallographic structure for the ferroelectric and paraelectric phases of this compound. Potential transferability is demonstrated by applying the parameters derived to the optimization of the known molecular structure for a similar inorganic system (rubidium tetrachlorozincate).

The architecture of Mg(II) centres in MAPO-36 solid acid catalysts

The local structure around Mg2+ ions of a Magnesium substituted aluminophosphate, with the ATS structure (MgAPO-36, Mg/P=0.08), in the as-prepared and calcined state has been investigated by Mg K-edge XAS spectroscopy. High quality XAS data were collected using the solid-state fluorescence detector. Mg2+ is found to replace tetrahedrally co-ordinated Al3+ in the as-prepared state and remained intact even after calcination, thus yielding a highly active, solid acid catalyst.

Effect of Si(IV) substitution on the stability of microporous ZnAPSO-44 solid acid catalysts: a combined XAS/XRD study

The combined XAS/XRD technique has been used to investigate the stability of the microporous structure during calcination process for a series of ZnAPSO-44 materials. Incorporation of large amounts of Zn is found to produce materials that are unstable upon removal of the structure directing organic template molecule. However, simultaneous incorporation of Si is found to increase the thermal stability of the materials, yielding catalysts that are well suited for acid catalysed reactions.

Structure refinement of quaternary spinel oxides experiments and modelling

Comprehensive studies have been undertaken, inclusive of experimental and computational techniques, on the structure and cation distribution of spinel solid solutions formed between the normal spinel LiMn2O4 and inverse LiFe5O8. Series of solid solutions of a composition (1 - x)LiMn2O4 x xLi0.5Fe2.5O4 are single phase products with spinel structure in the whole range of x, displaying a cubic structure. With increasing Fe3+ content, the tendency of ordering by lithium ions in octahedral spinel sites and a strongly marked preference of Li+ cations to occupy the octahedral positions is apparent. Modelling and refinement of crystal structure of such spinel solid solution series have been undertaken by the energy minimisation procedure, together with the interatomic potentials calculation, explaining some divergences of the experimental data.

Relationship of crystal structure to interionic interactions in the lithium-manganese spinel oxides

Lithium manganese oxides in the form of cubic spinel phases (space group Fd3m) are formed in a LixMn3-xO4 system for rather limited values of x. Structural investigations by X-ray powder diffraction, applied to the Li-Mn-O compounds, indicate the formation of a second crystalline phase, Li2MnO3 (space group C2/m), with the increasing lithium content. Total Li+ content per unit cell and the cation distribution over a spinel lattice in LixMn3-xO4 have been studied by measurements of integrated intensities of X-ray reflections, and by structure refinement using Rietveld profile analysis. In an attempt to understand the factors affecting cation distribution in the spinel lattice, we applied the computer modelling techniques and investigated the Li+, Mn3+ and Mn4+ ion distribution by calculating the lattice energy, combined with energy minimisation procedures, using the General Utility Lattice Program (GULP), a program designed for simulation of ionic and semi-ionic solids, based on interatomic potential models.

A New High-Flux Chemical and Materials Crystallography Station at the SRS Daresbury. 1. Design, Construction and Test Results

A new single-crystal diffraction facility has been constructed on beamline 9 of the SRS at Daresbury Laboratory for the study of structural problems in chemistry and materials science. The station utilizes up to 3.8 mrad horizontally from the 5 T wiggler magnet which can be focused horizontally and vertically. The horizontal focusing is provided by a choice of gallium-cooled triangular bent Si (111) or Si (220) monochromators, giving a wavelength range from 0.3 to 1.5 A. Focusing in the vertical plane is achieved by a cylindrically bent zerodur mirror with a 300 mum-thick palladium coating. The station is equipped with a modified Enraf-Nonius CAD-4 four-circle diffractometer and a Siemens SMART CCD area-detector system. High- and low-temperature facilities are available to cover the temperature range from about 80 to 1000 K. Early results on test compounds without optimization of the beam optics demonstrate that excellent refined structures can be obtained from samples giving diffraction patterns too weak to be measured with conventional laboratory X-ray sources, fulfilling a major objective of the project.

A New High-Flux Chemical and Materials Crystallography Station at the SRS Daresbury. 1. Design, Construction and Test Results

A new single-crystal diffraction facility has been constructed on beamline 9 of the SRS at Daresbury Laboratory for the study of structural problems in chemistry and materials science. The station utilizes up to 3.8 mrad horizontally from the 5 T wiggler magnet which can be focused horizontally and vertically. The horizontal focusing is provided by a choice of gallium-cooled triangular bent Si (111) or Si (220) monochromators, giving a wavelength range from 0.3 to 1.5 A. Focusing in the vertical plane is achieved by a cylindrically bent zerodur mirror with a 300 mum-thick palladium coating. The station is equipped with a modified Enraf-Nonius CAD-4 four-circle diffractometer and a Siemens SMART CCD area-detector system. High- and low-temperature facilities are available to cover the temperature range from about 80 to 1000 K. Early results on test compounds without optimization of the beam optics demonstrate that excellent refined structures can be obtained from samples giving diffraction patterns too weak to be measured with conventional laboratory X-ray sources, fulfilling a major objective of the project.

Strategies for modelling of catalysts

The strategy summarised above in which modelling calculations are combined with QM cluster calculations is a viable procedure for the study of catalysis. Our discussion has emphasized studies of zeolite catalysts but the same approach could be used in modelling reactions catalysed on metal and metal oxide surfaces. Moreover, the techniques and potentials are available for these methods to have a wide range of applications.

The conformation of apamin

Energy minimization techniques are used as a tool to distinguish between different proposed models for the structure of the bee venom polypeptide apamin. The influence of electrostatic interactions on the resultant energies is noted. The model of Hider and Ragnarsson [(1980) FEBS Lett. 111, 189-193] is found to be of consistently low energy.