Kjersti Gjønnes

On the integration of electron diffraction intensities in the Vincent-Midgley precession technique

Abstract In the Vincent-Midgley technique, where the electron beam is precessed around the optical axis, integrated electron diffraction intensities can be measured directly, similar to measurements performed in X-ray diffraction methods. When introduced into the structure determination or refinement schemes, the experimental intensities must be corrected for geometrical factors associated with the precession by a factor analogous to the Lorentz factor used in X-ray diffraction. In this paper, Lorentz factors are derived for reflections in zeroth- and higher-order Laue zones. The result differs from previously published correction factors (Vincent and Midgley, 1994 Ultramicroscopy 55 (1994) 271), the effect on structure determination or refinement being most important where reflections corresponding to a large range of lattice spacings are used. The effect of beam divergence is discussed and appropriate Lorentz factors derived.

Two-beam features in electron diffraction patterns – application to refinement of low-order structure factors in GaAs

Progress in theoretical approaches to the refinement of structure factors and structure determination by electron diffraction is briefly reviewed. A strategy based on the use of two-beam-like features in convergent-beam electron diffraction (CBED) patterns is advocated, based on an effective potential and a reduced excitation error. Model calculations suggest inversion of the Bethe potentials to be useful. New experimental measurements for CBED rocking curves in GaAs are reported and these give the refined values V(200) = 0.432 (9), V(400) = 4.53 (4) V at 90 K, including Debye-Waller factor. The results from the inverse Bethe potential method are in excellent agreement with full many-beam calculations. These experimental values are compared with the results of recent pseudopotential total-energy calculations.

Distribution of holes in YBa2Cu3O7 from convergent beam electron diffraction

Abstract A new method based on the dynamical coupling between high- and low-order reflections in convergent beam electron diffraction has been used to determine the structure factors for the (001) and (002) reflections in YBa2Cu3O6.95 ± 0.05. These low-order structure factors are very sensitive to the charge distribution. The best fit to experimental diffraction intensities was obtained with 3 5 of the holes in the CuO chains and 2 5 of the holes in the CuO2 planes.

Refinement of temperature factors and charge distributions in YBa2Cu3O7 and YBa2(Cu, Co)3O7 from CBED intensities

Abstract Wide angle convergent beam electron diffraction patterns (CBED) from YBa 2 Cu 3 O 6.9+0.1 and YBa 2 (Cu 0.78 Co 0.22 ) 3 O 6.9±0.1 have been used for refinement of temperature factors and for determination of ionicities. Least square refinement of experimental intensity profiles to calculations for reflections in the range (0 0 20) to (0 0 36) confirms the coordinates to within 0.005A. Temperature factors for Cu and Ba atoms have been determined, and indications of s -dependence in the temperature factors for Ba and Cu are found, which may be interpreted in terms of small displacement or anharmonic oscillations. Ionicities has been determined from the dynamic scattering between reflections in the range (0 0 15) to (0 0 25), and the distribution of holes between the CuO chains and the CuO 2 planes has been determined. The best fit was obtained with 40% and 20% of the holes in the CuO 2 planes for YBa 2 Cu 3 O 7 and YBa 2 (Cu 0.78 Co 0.22 )O 7 respectively.

Bloch wave symmetries and inclined surfaces

Abstract The Bloch wave formulation is derived from the integral equation for scattering, in such a way as to separate forward scattering and backward scattering. The orientation of the entrance surface is specified at the outset. This leads to clear definitions in reciprocal space, particularly of an excitation error along the normal to the entrance surface. Application to inclined entrance surface shows the effect of inclination on the Bloch wave amplitudes to be small in most practical cases. Situations where the amplitudes differ by a few percent from the values at normal incidence can be found, however.

Scale structures during oxidation and sulphidation of nickel and a nickel-chromium alloy

Abstract Simultaneous sulphidation and oxidation of nickel and Ni-20Cr has been studied by optical, scanning and transmission electron microscopy. The main constituents of the scales as revealed by SAD, CBED and EDS analysis are NiO and Ni3S2, which appear to be formed predominantly between the NiO grains. EDS indicates presence of sulphur also within the NiO grains. A superstructure of unknown composition was found in regions with low sulphur content. A method for absorption correction in EDS analysis which does not require thickness determination was tried out.

Determination of electrostatic potentials and charge distributions in bulk and at interfaces by electron microscopy techniques

High energy electron diffraction is sensitive to details in charge and potential distribution since scattering occurs by the net charge or electrostatic potential. Several techniques have been developped for characterization the atomic and electronic structure at or near interfaces: convergent beam profiles critical voltage effect in electron diffraction reflection/refraction at interfaces. Appplications are shown to bulk semiconductors single interfaces and multilayers.