David Keen

Abstract:

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.

Abstract:

The local structure of the low-temperature ordered phase of the negative thermal expansion (NTE) material ZrW2O8 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 WO4 and ZrO6 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. © 2005 The American Physical Society.

Abstract:

We describe a model-independent evaluation of the possibility of extracting detailed lattice dynamical information from neutron powder diffraction data. The method, which we first reported recently [Phys. Rev. Lett. 93, 075502 (2004)], is extended to include consideration of crystal symmetry. This is exploited to reduce errors in the approach and to allow unambiguous assignment of normal modes. The experimental and computational requirements are explored, with particular attention to the use of the reverse Monte Carlo method for generating atomistic configurations. Phonon-dispersion curves, extracted from neutron total scattering data using the technique, are presented for magnesium oxide and strontium titanate. These are used to illustrate the sensitivity and limitations of this method. © 2005 The American Physical Society.