Showing papers on "Electron backscatter diffraction published in 1990"

Cryo-protection of protein crystals against radiation damage in electron and X-ray diffraction

Abstract: By using the fading of electron diffraction patterns during electron irradiation of protein or other organic crystals as a benchmark to measure destruction of the crystalline atomic arrangement by ionizing radiation, calculations suggest that protein crystals in laboratory X-ray beams might last for about five years when the specimen is cooled to liquid nitrogen temperatures or below. It is suggested that all crystals should be equally stable to X-irradiation at this temperature, as they are to electron irradiation. The calculation, which depends on the assumption that electrons and X-rays are more or less equally damaging to the structure giving rise to the diffraction at very low temperature, supports experimental observations that X-ray diffraction from protein crystals seems to last indefinitely at liquid nitrogen temperatures, even in the most powerful beams available at synchrotron X-ray sources. An attempt is made to explain the relation between the present analysis and other ways of viewing radiation damage.

High-resolution transmission electron microscopy and electron diffraction of mixed-layer illite/smectite; experimental results

Abstract: High-resolution transmission electron microscopy (HRTEM) and electron diffraction experiments have been performed on R1 and R> 1 illite/smectite (I/S) samples that from X-ray powder diffraction (XRD) experiments appear to contain well-ordered layer sequences. The HRTEM images confirmed earlier computer image simulations, which suggested that periodicities due to I/S ordering can be imaged in TEM instruments of moderate resolution. The experiments also confirmed that in instruments of this sort, the strongest contrast arising from the compositional difference between I and S layers occurs under rather unusual imaging conditions of strong oveffocus. Some selected-area electron diffraction (SAD) patterns showed additional diffraction spots consistent with R1 and R3 ordering. SAD patterns and cross-fringes arising in HRTEM images from non-00l reciprocal lattice rows indicated that the stacking vectors of most adjacent 2:1 layers were not randomly oriented with respect to each other. Thus, the I/S was not fully turbostratic, but instead consisted of very thin, coherently stacked crystallites that extended across the fundamental particles postulated by Nadeau and coworkers. S/(I + S) ratios were determined for about seventy HRTEM images obtained and interpreted by three different TEM operators. These ratios were consistent with those obtained from standard XRD procedures, suggesting that results obtained by XRD can be used to infer the initial structural state of mixed-layer I/S prior to treatment of samples for XRD experiments. The HRTEM experiments thus demonstrated that the two specimens examined consisted of ordered I/S existing as small crystals, most of which contained more layers than the fundamental particles of Nadeau and coworkers. The non-turbostratic stacking suggests an energetic interaction between the individual fundamental particles, leading to at least two alternative thermodynamic descriptions of these materials. Although the I/S crystals in the present experiments probably were disaggregated into fundamental particles during sample preparation for XRD, the I/S crystals appear to have separated only along the smectite interlayers. If the term “fundamental particle” is to be used for primary, untreated I/S, its original definition should be modified to include not only free particles, but also those that occur as layers within small crystals. It further should be recognized that these particles can interact thermodynamically and crystallographically with their neighbors.

Structural Analysis of Orthorhombic ZrO2 by High Resolution Neutron Powder Diffraction

Abstract: A time-of-flight neutron diffraction experiment was undertaken on a powder sample of orthorhombic ZrO2 synthesized at 600°C and 6GPa for 30min. The Rietveld analysis for the diffraction data shows that the space group of this phase is Pbca. The refined lattice parameters are a0=1.00861, b0=0.52615, and c0=0.50910nm.

Texture characterisation of sputtered MoS2 thin films by cross-sectional TEM analysis

Abstract: Cross sections of RF sputtered MoS2 thin films are investigated using lattice resolution transmission electron microscopy and electron diffraction. The results are compared with X-ray diffraction measurements. The films have a (001) texture (type II) in the first nanometres near the interface and a conventional (100) texture (type I) in the upper part.

An assessment of multiple-scattering effects in Auger-electron diffraction and photoelectron diffraction

Abstract: Multiple-scattering effects in Auger electron diffraction and photoelectron diffraction are assessed using a new spherical-wave multiple scattering formalism which is based on a separable approximation to the scattering Green's function. Results obtained in both single-scattering and fully-converged multiple-scattering limits are presented for linear chains of Cu atoms. Simulations of some Auger electron diffraction and photoelectron diffraction experimental results with this new theory are also presented for the first time.

Neutron diffraction and transmission electron microscopy study of hydrogen‐induced phase transformations in Zr3Al

Abstract: Hydrogen‐induced phase transformations of the equilibrium intermetallic compound Zr3 Al have been studied by in situ elastic neutron scattering, x‐ray diffraction, and transmission electron microscopy (TEM). TEM observations reveal two distinct modes of amorphous phase formation in Zr3 Al upon hydrogenation, namely, heterogeneous nucleation at preexisting grain boundaries, and homogeneous nucleation within single‐crystal grains. In situ neutron diffraction reveals a phase separation between a hydrogen (deuterium) poor and a hydrogen (deuterium) rich crystalline phase. Rietveld profile refinement of the neutron diffraction data indicates predominant hydrogen (deuterium) occupation of the octahedral interstitial sites in the crystalline matrix that have only Zr nearest‐neighbor atoms. The two different modes of amorphous phase nucleation are directly related to the degree of hydrogen dissolution in the host crystalline matrix.

The influence of diffraction effects on quantitative Auger electron spectroscopy

Abstract: In Auger electron spectroscopy, both the primary electrons entering the sample and the Auger electrons leaving the sample may suffer strong diffraction effects. On well-ordered single-crystal surfaces, diffraction effects may result in variations in the intensity of an Auger peak by a factor of up to two or more when the incident beam direction is varied by a few degrees. The large angular acceptance angle of most Auger spectrometers normally averages out any diffraction effects of the Auger electrons themselves. Diffraction effects of the primary electron beam (commonly called channelling) may be described qualitatively in terms of the two-beam theory of electron diffraction. A more exact representation requires a multiple beam calculation similar to that used in electron microscopy. For quantitative AES it is essential to establish whether diffraction is likely to influence the results and, is so, to take appropriate steps to eliminate or at least to minimize its effect. On the other hand, these channelling effects provide valuable information about the near-surface order of the sample and potentially provide a technique complementary to x-ray photoelectron diffraction.

Phase identification of 140 K superconducting Bi2Sr2Ca2Cu3O10-δ doped with Sb and Pb

Abstract: The Tc=140 K superconductor Bi2Sr2Ca2Cu3O10- delta doped with Sb and Pb was characterised by powder X-ray diffraction, electron diffraction and electron probe microanalysis. It was observed in the specimen that there was a new phase Bi3Sb0.8Pb0.2Sr4.1Ca3.9CuO15+ delta besides 2212, 2223 and a little CuO. The new phase belongs to the monoclinic system with cell dimensions a=22.14, b=5.902, c=19.91 AA and beta =99.7 degrees .

Measurement of Local Textures With Transmission and Scanning Electron Microscopes

Abstract: Transmission and scanning electron microscopy methods are discussed for the determination of grain orientations. For the study of local textures with a TEM electron-transparent thin samples are required. The standard techniques of orientation determination grain by grain are the interpretation of selected area electron spot and microbeam Kikuchi diffraction patterns. Specimen areas smaller than 500 nm or 50 nm in diameter can be selected. More recently selected area pole-figures can be measured directly with a TEM technique similar to the conventional transmission X-ray method.The orientation of grains in a bulk sample can be obtained with a scanning electron microscope from reflection Kikuchi (i.e. electron backscattering) and channeling patterns. Local resolution is approximately 1 μm or 5 μm, respectively.Since the interpretation of electron diffraction patterns is tedious, techniques have been developed to perform measurements on-line by interfacing the electron microscope to a computer. An outstanding advantage of texture measurements by electron diffraction is the high local resolution and the ability of imaging the microstructure of the sampled region. Experimental results of individual grain-orientation measurements may be represented statistically by inverse pole-figures, orientation distribution functions and misorientation distribution functions.

X-ray and electron-diffraction study of single-crystal bi2sr2cacu2ox

Abstract: Using both x-ray and electron diffraction, we have studied the incommensurate modulation structure in single crystal Bi 2 Sr 2 CaCu 2 O x . Electron diffraction clearly indicates a small a ∗ -axis component of the modulation wave vector k , along with the more familiar b ∗ -axis component (0.004 a ∗ + 0.212 b ∗ ). This tilt of the k -vector breaks the mirror symmetry perpendicular to a- and b-axes as well as the twofold symmetry about a- and b-axes. The modulated structure thereby has only a mirror plane normal to the c-axis. Intense x-ray streaking along the c-axis at several satcllite reflections was considerably more pronounced than at the fundamental reflections, indicating a stacking disorder along the c-axis predominantly in the modulated structure.

Combined neutron and x-ray powder diffraction study of Fe 0.50 Co 0.48 V 0.02

Abstract: The large number of constituent elements and high symmetry of the ordered Fe-Co-V alloys make a complete solution of its structure impossible from a single diffraction experiment. We have carried out a Rietveld refinement of the site occupancies for a partially ordered Fe-Co-V alloy using both neutron and anomalous dispersion x-ray powder diffraction data in order to obtain a complete crystallographic solution to its structure. Contrary to previous assumptions that V occupies both sites randomly or magnetic and Moessbauer studies that suggest that V has a site preference for the Fe sublattice, we find that the V dopant preferentially occupies the Co sites.

Effect of heat treatment on the X-ray diffraction of Al-Cu-Fe quasicrystals

Abstract: X-ray powder diffraction peaks have been measured for melt-spun and annealed Al-Cu-Fe quasicrystals. From the change in width and position of the diffraction peaks it was found that the linear phason strain introduced by melt-quenching is removed by a 1 h isochronal anneal between 873 and 1093 K.

Atomic structure and morphology of the albite {010} surface: An atomic-force microscope and electron diffraction study

Abstract: The atomic structure and nanometer-scale morphology of the {010} surface of albite, exposed by fracturing in air at room temperature, has been studied with low-energy electron diffraction (LEED) and contact atomic-force microscopy (AFM). The LEED results suggest that this surface is very similar to a simple termination of the bulk structure; however, there is evidence that the surface structure exhibits slight lateral relaxation owing to modest shifts of surface atoms as they seek equilibrium positions in this low symmetry structure. The AFM used in this study has demonstrated a lateral resolution as low as ll0 nm depending on surface roughness (better lateral resolutions, in the range of0.1-l nm, are achievable) and a height resolution of 0.1 nm. Several reactive sites on the albite {010} surface have been imaged at this resolution, including very small pits and cleavage steps. These types of structure and morphological analyses on mineral surfaces have a direct application to studies dealing with mineral dissolution, sorption reactions on mineral surfaces, and fracture propagation.