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Jan 2004
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Abstract: Calcium phosphate cluster formation leads to the
nucleation of apatite. Apatite controls phosphorus availability
in the geosphere and is also an important biomineral (e.g.
bone, tooth). Formation of apatite at the atomic scale is
believed to take place via the formation of a Ca3
(PO4
)2
precursor. Very little is currently known about the nature and
evolution (in saturated aqueous solutions) of this precursor,
and how this may be affected/controlled by the presence of
organic species. Using the materials processing beamline, 6.2,
at Daresbury Synchrotron Radiation Source for in-situ
simultaneous small- and wide- angle X-ray scattering
(SAXS/WAXS) experiments, we were able to to monitor
apatite nucleation from supersaturated solutions at times
scales of up to 30 minutes and at 10 second resolution. Our
aim was to test the influence of a range of organic ligands
(citrate, oxalate, osteocalcin) on nucleation and growth. Our
preliminary observations suggest that organic ligands affect
both the onset and the rate of crystal formation and growth, as
well as the settling properties of apatite nucleating from a
supersaturated solution (Figure 1). We discuss our results in
the context of the precipitating apatite, as a proxy for
bioapatites
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Jan 2004
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Abstract: A new high-pressure single-crystal diffraction facility has been constructed on station 9.8 at the Synchrotron Radiation Source, Daresbury Laboratory, for a range of studies on a variety of systems of relevance to physics, chemistry and materials science that would otherwise prove intractable with conventional laboratory-based methods. The station has been equipped with a modified Enraf-Nonius CAD4 four-circle diffractometer for high-pressure studies which can be conveniently, and rapidly, interchanged with the Bruker SMART CCD area-detector system when more routine ambient-pressure diffraction work is to be undertaken. This rapid change-over has been achieved by permanently mounting the CAD4 on its own jacking table, formerly used for the station's white-beam diffraction mode, which allows the alignment of the SMART diffractometer to remain undisturbed when the CAD4 is in use. Early results on the test low-melting-point compound ethanol (CH3CH2OH) reveal that excellent refined structures can be obtained, including the location and refinement of the H atoms, demonstrating that one of the original, and major, objectives of the station has been accomplished.
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Jan 2004
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S.
Dhesi
,
A.
Mirone
,
C.
De Nadaï
,
P.
Ohresser
,
P.
Bencok
,
N
Brookes
,
P.
Reutler
,
A.
Revcolevschi
,
A.
Tagliaferri
,
O.
Toulemonde
,
G.
Van Der Laan
Abstract: Orbital ordering (OO) in the layered perovskite La0.5Sr1.5MnO4 has been investigated using the enhanced sensitivity of soft x-ray resonant diffraction at the Mn L edges. The energy dependence of an OO diffraction peak over the L2,3 edges is compared to ligand-field calculations allowing a distinction between the influences of Jahn-Teller distortions and spin correlations. The energy dependence of the diffraction peak at the Mn L1 edge is remarkably different from that observed at the Mn K edge.
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Feb 2004
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Abstract: We demonstrate a diffraction geometry which provides strong sensitivity to the microstructure of thin films. While a coherent beam of x rays is being reflected from the surface of the sample, measurements were made of the scattering of the exit beam below the critical angle for total external reflection.This results in a strong signal with speckle modulations that are characteristic of the internal arrangement of grains at different depths within the film.
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Mar 2004
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Abstract: In 1947 Kathleen Lonsdale conducted a series of experiments on X-ray diffraction using a divergent beam external to a crystal sample. Unlike the Kossel technique, where divergent X-rays are excited by the presence of fluorescing atoms within the crystal, the use of an external divergent source made it possible to study non-fluorescing crystals. The resulting photographs not only illustrated the complexity of X-ray diffraction from crystals in a truly beautiful way, but also demonstrated unprecedented experimental precision. This long-forgotten work is repeated here using a synchrotron radiation source and, once again, considerable merit is found in Lonsdale's technique. The results of this experiment suggest that, through the use of modern 'third-generation' synchrotron sources, divergent-beam diffraction could soon enjoy a renaissance for high-precision lattice-parameter determination and the study of crystal perfection.
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Mar 2004
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Abstract: FePd thin-film samples with different perpendicular magnetic anisotropies have been studied with magnetic force microscopy (MFM), micromagnetic calculations and soft x-ray resonant magnetic scattering (SXRMS). The competition between perpendicular magnetic anisotropy (PMA) and shape anisotropy leads to the formation of highly ordered stripe domain patterns with a magnetization component perpendicular to the film plane. The magnetic stripes with a period of similar to100 nm, which are seen in the MFM images and can be modeled by micromagnetic calculations, give rise to magnetic peaks in the diffraction pattern. Closure domains occur in samples with a low to medium PMA, while a high PMA inhibits their formation. The in-plane magnetization component of the closure domains is not observable with MFM. In the presence of closure domains the interference between the scattering amplitudes in SXRMS from perpendicular and in-plane magnetized domains gives rise to a circular dichroism in the transverse geometry, where the scattering plane is along the stripes. We also recorded the magnetic speckle pattern from an 8 mum FePd wire using coherent x-rays and CCD detection. A high degree of coherence was obtained as evidenced from the observed intensity fluctuations. The speckle pattern can in principle provide information about the local disorder of the magnetic stripe domains.
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Apr 2004
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Abstract: Soft X-ray resonant magnetic scattering studies on a magnetically modulated, but nominally chemically homogenous, 5 nm Ni layer in a Cu/Ni/Cu/Co/NiO/GaAs(110) system are reported. It was possible to estimate the main chemical structure of the sample on the basis of the results from specular reflectivity and rocking scans, probing the scattering vector components in the perpendicular and in-plane direction to the Ni wires, respectively. The magnetic scattering using polarized X-rays demonstrates the magnetic modulation of the Ni layer.
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May 2004
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Abstract: Secondary extinction effects in x-ray holography with internal atomic reference are examined in the frame of multiple-scattering power transfer equations for a mosaic crystal. The calculations are compared with experimental holograms of an imperfect Cu3Au single crystal measured using x-ray fluorescence and total electron yields. It is shown that extinction effects in imperfect crystals can influence reconstructed real-space images in x-ray atom-resolving holography.
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May 2004
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Abstract: One performed for the first time quantum and mechanical calculation of 115 forbidden Bragg reflection spectra in a wurtzite structure ZnO crystal. Occurrence of the forbidden reflection is caused by distortion of circle local symmetry of resonance atoms at the cost of thermal oscillations. On the basis of the FDMNES Program one simulated temperature effect on intensity amplification of 115 forbidden reflection in w-ZnO crystal. The dipole-quadrupole contribution into the intensity amplification of the forbidden reflections is found to be a rather essential one. The calculation results are in line with the previously obtained experimental data
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May 2004
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