Entity

Time filter

Source Type


Straaso T.,Copenhagen University | Dippel A.-C.,German Electron Synchrotron | Becker J.,Center for Materials Crystallography | Als-Nielsen J.,Copenhagen University
Journal of Synchrotron Radiation | Year: 2014

Under the experimental condition that all Bragg peaks in a powder X-ray diffraction (PXRD) pattern have the same shape, one can readily obtain the Bragg intensities without fitting any parameters. This condition is fulfilled at the P02.1 beamline at PETRA III using the seventh harmonic from a 23 mm-period undulator (60 keV) at a distance of 65 m. For grain sizes of the order of 1 μm, the Bragg peak shape in the PXRD is entirely determined by the diameter of the capillary containing the powder sample and the pixel size of the image plate detector, and consequently it is independent of the scattering angle. As an example, a diamond powder has been chosen and structure factors derived which are in accordance with those calculated from density functional theory methods of the WIEN2k package to within an accuracy that allows a detailed electron density analysis.© 2014 International Union of Crystallography. Source


Straas T.,Niels Bohr Institute | Becker J.,Center for Materials Crystallography | Iversen B.B.,Center for Materials Crystallography | Als-Nielsen J.,Niels Bohr Institute
Journal of Synchrotron Radiation | Year: 2013

In a powder diffraction pattern one measures the intensity of Miller-indexed Bragg peaks versus the wavevector transfer sin/. With increasing wavevector transfer the density of occurrence of Bragg peaks increases while their intensity decreases until they vanish into the background level. The lowest possible background level is that due to Compton scattering from the powder. A powder diffraction instrument has been designed and tested that yields this ideal low-background level, obtainable by having the space between sample and detector all in vacuum with the entrance window so far upstream that scattering from it is negligible. To minimize overlap of Bragg peaks the combination of fine collimation of synchrotron radiation, a thin cylindrical sample and a high-resolution imaging plate detector is taken advantage of. © 2013 International Union of Crystallography Printed in Singapore-all rights reserved. Source


Andersen H.L.,Center for Materials Crystallography | Christensen M.,Center for Materials Crystallography
Nanoscale | Year: 2015

The evolution of size and size distribution during hydrothermal synthesis of nanocrystalline CoFe2O4 has been studied by in situ synchrotron powder X-ray diffraction (PXRD). Varying synthesis temperature or [OH-] concentration in the precursor proves to have no significant effect on the final volume-weighted nanocrystallite sizes (∼12 nm) of CoFe2O4. However, analysis by whole powder pattern modeling of the [OH-] concentration series reveals a substantial difference in the number-weighted size distributions when varying the amount of base used. Furthermore, changing the metal ion concentration prior to NaOH addition in the precursor preparation gives a handle to control the nanoparticle sizes (∼5-15 nm). All in situ experiments show almost instantaneous formation of the CoFe2O4 nanocrystallites, without significant growth or broadening of the size distribution after 60 s. Magnetic hysteresis curve measurements illustrate, how this facilitates the tailoring of materials with specific magnetic properties, as larger particles (∼15 nm) exhibit hard magnetic properties while the smaller particles (∼6-7 nm) are superparamagnetic. This journal is © The Royal Society of Chemistry. Source


Bindzus N.,Center for Materials Crystallography | Straaso T.,Niels Bohr Institute | Wahlberg N.,Center for Materials Crystallography | Becker J.,Center for Materials Crystallography | And 4 more authors.
Acta Crystallographica Section A: Foundations and Advances | Year: 2014

Synchrotron powder X-ray diffraction data are used to determine the core electron deformation of diamond. Core shell contraction inherently linked to covalent bond formation is observed in close correspondence with theoretical predictions. Accordingly, a precise and physically sound reconstruction of the electron density in diamond necessitates the use of an extended multipolar model, which abandons the assumption of an inert core. The present investigation is facilitated by negligible model bias in the extraction of structure factors, which is accomplished by simultaneous multipolar and Rietveld refinement accurately determining an atomic displacement parameter (ADP) of 0.00181 (1)°2. The deconvolution of thermal motion is a critical step in experimental core electron polarization studies, and for diamond it is imperative to exploit the monatomic crystal structure by implementing Wilson plots in determination of the ADP. This empowers the electron-density analysis to precisely administer both the deconvolution of thermal motion and the employment of the extended multipolar model on an experimental basis. © 2014 International Union of Crystallography. Source


Norby P.,Center for Materials Crystallography | Jensen K.M.O.,Copenhagen University | Lock N.,University of Aarhus | Christensen M.,Center for Materials Crystallography | Iversen B.B.,Center for Materials Crystallography
Crystal Growth and Design | Year: 2016

Yttrium and ytterbium aluminum garnet (Y3Al5O12 and Yb3Al5O12) nanoparticles have been synthesized under sub- and supercritical water conditions in a continuous flow reactor. The particle size has been investigated by scanning and transmission electron microscopy. The heat-induced structural changes of the garnet structure have been investigated by multitemperature powder X-ray diffraction at a synchrotron source (100-1000 K). In combination, Rietveld refinement of these multitemperature data and thermal analysis indicate a proposed diffusion mechanism for the aluminum and yttrium/ytterbium atoms in the garnet structure, which leads to fewer defects at higher temperature. Hence, hydrothermally synthesized nanoparticles give novel knowledge about the disordered internal structure important for their use in optical applications. © 2016 American Chemical Society. Source

Discover hidden collaborations