Large well-relaxed models of vitreous silica, coordination numbers and entropy

A Monte Carlo method is presented for the simulation of vitreous silica. Well-relaxed networks of vitreous silica are generated containing up to 300,000 atoms. The resulting networks, quenched under the BKS potential, display smaller bond-angle variations and lower defect concentrations, as compared to networks generated with molecular dynamics. The total correlation functions T(r) of our networks are in excellent agreement with neutron scattering data, provided that thermal effects and the maximum inverse wavelength used in the experiment are included in the comparison. A procedure commonly used in experiments to obtain coordination numbers from scattering data is to fit peaks in rT(r) with a gaussian. We show that this procedure can easily produce incorrect results. Finally, we estimate the configurational entropy of vitreous silica.Comment: 7 pages, 4 figures (two column version to save paper

The effect of the annealing temperature on the local distortion of La0.67_{0.67}Ca0.33_{0.33}MnO3_3 thin films

Mn $K$-edge fluorescence data are presented for thin film samples (3000~\AA) of Colossal Magnetoresistive (CMR) La$_{0.67}$Ca$_{0.33}$MnO$_3$: as-deposited, and post-annealed at 1000 K and 1200 K. The local distortion is analyzed in terms of three contributions: static, phonon, and an extra, temperature-dependent, polaron term. The polaron distortion is very small for the as-deposited sample and increases with the annealing temperature. In contrast, the static distortion in the samples decreases with the annealing temperature. Although the local structure of the as-deposited sample shows very little temperature dependence, the change in resistivity with temperature is the largest of these three thin film samples. The as-deposited sample also has the highest magnetoresistance (MR), which indicates some other mechanism may also contribute to the transport properties of CMR samples. We also discuss the relationship between local distortion and the magnetization of the sample.Comment: 11 pages of Preprint format, 8 figures in one tar fil

AN X-RAY MONOCHROMATOR SUITABLE FOR STUDYING BIOLOGICAL SYSTEMS

An X-ray monochromator which is now in routine use at the EMBL Outstation in Hamburg is described. The device uses two separate (220) silicon crystals, the first of which is thermally isolated from the body of the monochromator. The relative angular setting of the two crystals is adjusted by a small piezoelectric stack mounted on a stiff beryllium-copper spring. Rapid settling (120 msecs) minimises the time lost for the monochromator to stabilise and hence X-ray beam damage on the specimen. The relative angular setting of the two crystals is controlled via signals from a photo-emission device and the first ionisation chamber. These signals are combined in a second order control system. By driving the monochromator piezoelectric with a periodic ramping signal, the performance and alignment of the device can be checked. The effects of a temperature difference between the crystals caused by beam heating is also discussed

PROGRESS IN SCF-SW-XALPHA AB INITIO XANES CALCULATIONS FOR CHROMIUM HEXACARBONYL BASED ON GENERAL NON-MUFFIN-TIN POTENTIALS

We describe progress towards the performance of SCF-SW-Xalpha calculations of photo-absorption cross-sections based on the theory of Natoli et al. for non-muffin-tin potentials. A crucial requirement is the accurate modelling of the electron-molecule potential using spherical harmonic expansions. We describe how this has been achieved and what difficulties are encountered. In the particular case of our model compound, chromium hexacarbonyl, we show what muffin-tin calculations produce and show that we may expect significant improvements from a non-muffin-tin calculation. Finally, we comment on the programming problems involved in these computations

MULTIPLE SCATTERING CALCULATIONS FOR BIOLOGICAL CATALYSTS

A restricted multiple scattering calculation based on the plane wave approximation but retaining Hankel function terms is presented for zinc bound to four imidazole ligands which is an excellent model for many enzyme active sites. The data show that multiple scattering calculations are essential to understand the EXAFS spectrum. It is also shown that Fourier transforms of this data are very sensitive to rotations about an axis perpendicular to the imidazole plane, and less so for out of plane rotations. The Fourier transform beyond shell one is not simply interpretable in terms of metal ligand distances owing to the interference of many multiple scattering paths

Inelastic X-Ray Scattering Measurements at BM29

An inelastic x-ray scattering spectrometer has been implemented on the bending magnet beamline BM29 at the ESRF. It comprises a spherically bent perfect crystal as a Bragg analyser in near back reflection and a linear position sensitive detector (PSD) in Rowland's geometry. The setup exploits the wide horizontal radiation fan from a dipole magnet to create an extended source so that several eV of scattered radiation are simultaneously imaged onto the PSD with a static analyser crystal. This is in contrast to existing configurations using undulator or wiggler sources where one component of the emission spectrum is imaged and the analyser has to be scanned in angle. Results are shown at the holmium Lm edge where the resolution is better than ~0.3 eV throughout the whole spectrum

EXAFS INVESTIGATION OF THE STRUCTURAL SITE OF LIVER ALCOHOL DEHYDROGENASE

Date are presented of the X-ray absorption near edge and EXAFS region for the Zinc environment of the structural site of horse liver alcohol dehydrogenase (HLADH). Using metal replacement or extraction, X-ray absorption measurements could be performed exclusively on the structural site. Measurements were performed at 20 K both on 3 types of enzyme (HLADH without catalytic centres, HLADH with coenzyme and HLADH with inhibitor bound to the active site) and on 2 model compounds namely cubic Zinc sulphide and Zinc dimethyldithiocarbamate. Both, the mean interatomic distance (Zn-S) and the thermal parameters indicate that the enzymes' local structure is most closely modelled by cubic Zinc sulphide

Anisotropy in Borrmann spectroscopy

In this paper we introduce Borrmann Spectroscopy as a method for measuring X-ray absorption spectra under conditions of an exotic wave field, namely, a coherent superposition of two plane waves. The essential features of the Borrmann Effect (also known as anomalous transmission) are outlined. We show that the Borrmann Effect can lead to a very strong relative enhancement of quadrupole absorption. After describing some early results in this field, and some general considerations of multiple-wave absorption, we contrast recent results on anisotropy in Borrmann spectroscopy with normal absorption. Despite the qualitative success of a simple model for quadrupole enhancement, temperature dependence and anisotropy, a general theory of the Borrmann Effect is required which includes anisotropic and non-dipolar scattering. We outline some first steps towards such a theory