A procedure for refining structural parameters and charge densities using convergent-beam electron diffraction (CBED) is described. The procedure is based on the nonlinear least-squares fitting between full dynamical calculations and energy-filtered intensities of two-dimensional higher-order Laue-zone (HOLZ) and zeroth-order Laue-zone (ZOLZ) CBED patterns.
Analysis methods of electron energy-loss spectroscopy (EELS) based on transmission electron microscopy are described. An introduction includes a short history, types of spectrometers and advantageous points of EELS. Thickness determination and elemental analysis methods are briefly described. Information of electronic structures appearing in valence-loss spectra is explained using spectra of graphite, C60 and diamond. Characteristic effects on core-loss spectra of the selection rule, on site excitation, chemical shift and core-hole interaction are explained.
The crystal structure of spinel compound CuIr2S4 was investigated by combination of high-resolution x-ray, electron and neutron diffraction measurements. CuIr2S4 is well known to show metal-insulator (MI) transition accompanied by structural transition at TMI-230 K. The electrical resitivity abruptly increases at TMI, on cooling and the localized magnetic moment vanishes simultaneously. In the present study, the crystal structure below TMI is successfully determined. The results show that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition, which can explain the physical properties of CuIr2S4. The charge-ordering pattern consists of isomorphic Ir83++S24 and Ir84++S24. This arrangement is a new description of the spinel structure but much more complex than the known charge-ordering patterns such as stripes.
The crystallographic superstructures were investigated for charge-orbital ordered manganites, Nd1-xSr1+xMnO4 (x=2/3, 3/4), by low-temperature high-resolution electron microscopy. Transverse and sinusoidal structural modulations were successfully observed in the images at 80 K. The observation implies that a new kind of“Wigner-crystal”model accompanied with a charge-density wave of eg electrons best explains the charge-orbital ordered state.
Hole concentration dependence of a charge order and an orbital order in layered Perovskite compounds La1-xSr1+dxMnO4 has been studied by means of the resonant X-ray scattering technique. Both orders have been observed in the compounds having the hole concentration x between 0.5 and 0.48, and not observed for x≤0.46. The x dependence of the correlation length suggest that the orbital order is prerequisite to the forming of the charge order in this system.
Direct space methods are powerful techniques for solving crystal structures from powder diffraction data, in particular, when observed intensity data have low angular resolution. In the present article, basic ideas of direct space methods using Monte Carlo methods and simulated annealing are described. Various techniques for model buildings to be used for searching a structural solution in direct space are presented. Examples of application of the methods to real materials are given.
The most important purpose of Rietveld analysis is the refinement of lattice and structure parameters. In addition, we can obtain geometric data (in turn, bond valence sums), magnetic moments (neutron diffraction), crystallite sizes, microstrains, and mass fractions. They are briefly described so that one can gain a better understanding of the information extractable from powder diffraction data.