Biomedical subjects

Andrew L Goodwin

Publications and source records attributed to Andrew L Goodwin.

4 recordsLinked to original sources

PubMed2006

Magnetic structure of MnO at 10 K from total neutron scattering data.

Total neutron scattering data from a powdered sample of MnO collected at 10 K have been analyzed using the reverse Monte Carlo method to refine the nuclear and magnetic structure. The results give the first unambiguous assignment of the average magnetic structure. The magnetic moments are aligned ferromagnetically within (111) sheets with the magnetization vectors of alternate sheets along axes parallel and antiparallel to the <112> directions, albeit with a small modulated out-of-plane component. Small displacements of Mn and O (modulated with the same periodicity) accompany the magnetic ordering and both atomic and magnetic structures may be described in the monoclinic space group C2.

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PubMed2005

Guest-dependent negative thermal expansion in nanoporous prussian blue analogues M(II)Pt(IV)(CN)6.x{H2O} (0 < or = x < or = 2; M = Zn, Cd).

The guest-dependent thermal expansion behavior of the nanoporous Prussian Blue analogues MIIPtIV(CN)6.x{H2O} (0 </= x </= 2; M = Zn, Cd) has been investigated using variable temperature single-crystal X-ray diffraction. The dehydrated phases MIIPtIV(CN)6 were found to exhibit negative thermal expansion, attributed to thermal population of low energy transverse vibrations of the bridging cyanide ligands. The presence of guest molecules within the framework pore system was found capable of dampening the effect of these transverse vibrational modes. The guest-loaded ZnII phase, in which the available pore volume is commensurate with the volume occupied by individual water molecules, possesses a considerably higher coefficient of thermal expansion, with the material switching from positive to negative thermal expansion behavior upon guest removal.

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PubMed2005

Negative thermal expansion in ZrW2O8: mechanisms, rigid unit modes, and neutron total scattering.

The local structure of the low-temperature ordered phase of the negative thermal expansion (NTE) material has been investigated by reverse Monte Carlo (RMC) modeling of neutron total scattering data. We obtain, for the first time, quantitative measurements of the extent to which the and polyhedra move as rigid units, and we show that these values are consistent with the predictions of rigid unit mode theory. We suggest that rigid unit modes are associated with the NTE. Our results do not support a recent interpretation of x-ray-absorption fine structure spectroscopy data in terms of a larger rigid structural component involving the Zr-O-W linkage.

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PubMed2004

Phonons from powder diffraction: a quantitative model-independent evaluation.

We describe a model-independent approach for the extraction of detailed lattice dynamics information from neutron powder diffraction data, based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement. Phonon dispersion curves for MgO extracted in this way are shown to reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. By means of molecular dynamics simulations, we quantify the extent to which the diffraction data are sensitive to lattice dynamics in this system.

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