Showing papers on "Electron backscatter diffraction published in 1980"

Charge density in anhydrite, CaSO4, from X-ray and neutron diffraction measurements

Abstract: The room-temperature electron-density distribution in natural anhydrite, CaSO4, has been studied by both X-ray and neutron diffraction. Conventional refinements of the data sets yielded RXw -0.020 and RNw = 0.025. The X-ray intensities were also used for high-order refinements, a refinement based on a point-charge model for the representation of positive residual charge accumulations, and a multipole expansion refinement. All these refinements were performed to study and to reduce parameter bias arising from bonding effects. The multipole expansion model yielded the significantly improved R w = 0.0116. The resulting (X--X) and (X-N) dynamic deformation density maps showed features qualitatively in agreement. Special attention was given to the charge distribution within and around the [SO4] 2anion. The observed bond density distribution is different from earlier results obtained from comparable sulfate derivatives and from the [8206 ]2anion, indicating a considerable 3d population of the S atom.

Electron diffraction intensities from bent molecular organic crystals

Abstract: The mosaic model for thin molecular organic microcrystals does not adequately account for the electron diffraction intensities from them; nor is two-beam dynamical theory adequate. A bent-crystal model for diffraction proposed by Cowley [Acta Cryst. (1961), 14, 920-927] explains well the observed electron diffraction intensities from orthorhombic paraffin microcrystals. In addition, previous verifications of n-beam dynamical electron scattering from these crystals (by fit of phase-grating structure-factor moduli to observed data and by the nonexistence of kinematical data at small electron wavelengths) are confirmed further by a good correspondence between calculated and observed n-beam data for increasing crystal thickness (i.e. monolayers of increasing-chain-length paraffins) and the change of continuously excited reciprocal-row intensities as the crystal is rotated about the axis. The better model for thin molecular organic microcrystals is therefore that of an elastically deformed perfect-crystal foil. Although n-beam dynamical effects can be demonstrated, the major limitation to crystal-structure analysis using intensity data from thin organic crystals is caused by the elastic bendings. This is particularly true if the crystals are solution grown and the longest unit-cell direction is therefore parallel to the incident beam. Epitaxial crystal growth, which minimizes bend effects to intensity data by forcing a shorter unit-cell axis to parallel the incident beam, effectively removes this apparent coherence restriction.

Electron Diffraction and Microscopy Studies of the Structure of Grain Boundaries in Al2O3

Abstract: Electron diffraction and weak-beam imaging techniques were used to examine the structure and thickness of grain boundaries in polycrystalline Al2O3. Extra diffraction spots from boundaries inclined to the foil surface were detected and related to the periodic structure of the boundary. Relrods from edge-on boundaries were detected and used to estimate the thickness of the boundary region, i.e. the depth that the displacement field of the boundary penetrates into the two crystals adjacent to the interface.

X-Ray Diffraction Study of a Concentrated Al(NO3 )3 Solution

Abstract: Abstract The scattering of X-rays from a concentrated Al (NO3)3 aqueous solution has been measured and analyzed at 25 °C. Good agreement with experimental data is achieved through a model in which Al3+(H2O)6 interacts with twelve water molecules through short and linear H-bonds. The water molecules which are the nearest neighbors to the Al3+ ions have trigonal orientation and each 0 atom in the nitrate ion gives rise to about 2 weak hydrogen bonds with water molecules.

Influence of Doping on the Crystal Potential of Silicon investigated by the Convergent Beam Electron Diffraction Technique

Abstract: Abstract Low index structure potentials of silicon were determined by convergent beam electron diffraction (Kossel-Möllenstedt technique) from very small crystal areas of about 100 Å in diameter. The values of 111, 222, 220, 113 and 004, determined to an accuracy of ±0.03 volts, are in excellent agreement with the accurate X-ray results of Aldred and Hart (see [6], p. 239). Heavy arsenic or phosphorous doping was found to cause a shift of 0.15 volts in the 111 structure potential. Absorption potentials were also determined and found to be 1/3 of the theoretical values published by Radi [20].

A neutron diffraction refinement of the crystal structure of erythritol at 22.6 K

Abstract: Erythritol, 1,2,3,4-butanetetrol (R*,S*), CAS No. 31904-60-6, C4H1004, M r = 122.08, I41/a, a = 12.713 (5), c = 6.747 (2)/k at 22.6 K, Z = 4, D c = 1.487 Mg m -3, molecular symmetry 1. The structure has been refined with 1077 neutron diffraction data measured at 22.6 K to an R(F 2) of 0.041. The molecules in the crystal have orientational disorder about the C(2)-O(2) bond with H(O2) having 15% occupancy of an alternate site. Rigid-body analyses, based on the non-hydrogen atoms and on all atoms except the hydroxyl H atoms gave excellent fits, with r.m.s. AUl/s of 0.0004 and 0.0006/k 2 respectively. The bond lengths corrected for librational motion are C(1)-C(2), 1.524 (1); C(2)-C(2 ') , 1.541 (1); C(1) O(1), 1.427 (I); C(2)-O(2), 1.435(1)/k. The bond lengths corrected for riding motion are C(1)-H(C11), 1.114(1); C(1)-H(C12), 1.116(1); C(2)-H(C2), 1.117(1); O(1 ) -n (o1 ) , 0.999(1); O(2)-H(O2), 0.985(1); O(2)--H(O2)*, 0 .923(1)A. The H . . . O hydrogen-bond lengths and O H . . . O angles (uncorrected) are 1.699 (2)/k, 173.3 (2)°; 1.788 (2) A, 164.5 (2) °. The shorter O ( 1 ) H . . . O ( 1 ) bonds form infinite helical chains about the 41 axes; the longer O ( 2 ) H . . . O ( 2 ) bonds form closed quadrilaterals about the 4 axes. The minor components of the disordered H atom form the same quadrilaterals about the 4 axes in the opposite direction.

The study of grain boundary thickness using electron diffraction techniques

Abstract: An electron diffraction technique has been developed to study the thickness of grain boundaries in polycrystalline specimens. Grain boundaries with a periodic structure in Al2O3, Cu-Bi and Au, were examined in an edge-on orientation using electron microscopy and diffraction. Each boundary gave rise to streaks in the diffraction pattern, the length of which was inversely related to the thickness of the boundary. For each boundary studied the magnitude of the boundary thickness was similar to the periodicity of the boundary, as determined from the image and the diffraction pattern.

Transmission electron microscopy and neutron diffraction studies of Feti-H(D)☆

Abstract: We present the results of a transmission electron microscopy (TEM) study of hydride precipitation in FeTi. The β and γ phases could be directly examined without loss of hydrogen. Cracking was mainly observed on dehydriding. Structural data on the β and γ phases obtained by TEM and neutron diffraction are presented and discussed. Some evidence was found for the existence of a third ordered hydride phase. We present a hypothetical FeTi-H phase diagram which is in agreement with the experimental results of various workers. A short analysis is given of the lattice expansion due to the formation of the hydride phases.

Transmission electron diffraction and microscopy of synthetic high pressure MgSiO3 phase with perovskite structure

Abstract: Synthetic perovskite (Mg, Fe) SiO3 prepared in a diamond anvil high pressure cell at 375 kbar and 1000°C has been examined in transmission electron microscopy and selected area diffraction. The crystals grow, probably by nucleation and growth, as small lozenge platelets on {111} planes of the pseudo cubic ideal cell. The diffraction patterns are compatible with a pseudo tetragonal cell with parameters twice as large as those previously determined by X-ray diffraction.

Symmetry determination of the room-temperature form of LnNbO4 (Ln = La,Nd) by convergent-beam electron diffraction

Abstract: Three different space groups, C2/c, C2 and Cc, have been reported for the room-temperature form of the rare-earth orthoniobates, LnNbO4 (Ln = La, Nd). They belong to different point groups: 2/m, 2 and m, respectively. By means of convergent-beam electron diffraction, the point group has been determined to be 2/m in the present study. Therefore the true space group must be C2/c.

Symmetry of the Au (110) surface reconstruction studied by spin‐polarized low‐energy electron diffraction

Abstract: Spin‐polarized low‐energy electron diffraction measurements of the analyzing power of two specular beams and the (0, 1/2 ) beam are presented. A theoretial argument relates the analyzing power S and polarizing power P of these beams for an unreconstructed Au (110) surface. This is used to deduce the symmetry of the reconstructed surface. In particualr, these results are inconsistent with the missing‐ and pairing‐row models, suggesting rotation within the reconstructed unit cell.

The space‐group determination of GaS and Cu3As2S3I by convergent‐beam electron diffraction

Abstract: Space-group determinations were carried out on GaS and Cu3As2S3I single crystals using convergent-beam electron diffraction data. The space groups found were P63/mmc and Cmcm, respectively. The effectiveness of the specific test for centrosymmetry was examined. In both structures the test was made clearer by use of the inclined-axis technique, in which the incident beam is at an appreciable angle to the principal zone axis. It was concluded that the test was superior to optical methods in structures such as GaS which have a high dislocation density.