X-ray and neutron scattering

Abstract:

The influence of electrostatic interactions on the ordering of sodium ions in NaxCoO2 is studied theoretically through Monte-Carlo simulations. For large x small di- or tri-vacancy clusters are stable with respect to isolated Na vacancies. At commensurate fillings these small clusters order in triangular superstructures. These results agree with recent electron diffraction data at x = 1/2 and 3/4. We have performed neutron Laue diffraction experiments at higher x, which confirm the predictions of this simple model. The consequences on the properties of the electronic charges in the Co layers are discussed.

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 report polarized- and unpolarized-neutron inelastic-scattering measurements of the magnetic excitation spectrum in the spin-charge ordered phase of La3/2 Sr1/2 Ni O4. Up to energies of ∼30 meV we observe broad magnetic modes characteristic of a near checkerboard ordering. A linear spin-wave model for an ideal checkerboard ordering with a single antiferromagnetic exchange interaction J′ =5.8±0.5 meV between next-nearest-neighbor spins on Ni2+ sites, together with a small XY -like single-ion anisotropy, provides a reasonable description of the measured dispersion. Above 30 meV the excitations are not fully consistent with the linear spin-wave model, with modes near the two-dimensional reciprocal space wave vector (0.5, 0.5) having an anomalously large intensity. Furthermore, two additional dispersive modes not predicted by spin-wave theory were observed, both of which are probably magnetic. One disperses away from (0.5, 0.5) in the energy range between 50-56 meV, and the other appears around (h,k) type positions (h,k=integer) in the energy range 31-39 meV. We propose a model in which these anomalous features are explained by the existence of discommensurations in the checkerboard ordering. At low energies there is additional diffuse scattering centred on the magnetic ordering wave vector. We associate this diffuse scattering with dynamic antiferromagnetic correlations between spins attached to the doped holes. © 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.