Compositional Analysis of III-V Interface Lattice Images

Since the point resolution of the JEOL 200CX electron microscope is up = 2.6Å it is not possible to obtain a true structure image of any of the III-V or elemental semiconductors with this machine. Since the information resolution limit set by electronic instability (1) u0 = (2/πλΔ)½ = 1.4Å for Δ = 50Å, it is however possible to obtain, by choice of focus and thickness, clear lattice images both resembling (see figure 2(b)), and not resembling, the true crystal structure (see (2) for an example of a Fourier image which is structurally incorrect). The crucial difficulty in using the information between Up and u0 is the fractional accuracy with which Af and Cs must be determined, and these accuracies Δff/4Δf = (2λu2Δf)-1 and ΔCS/CS = (λ3u4Cs)-1 (for a π/4 phase change, Δff the Fourier image period) are strongly dependent on spatial frequency u. Note that ΔCs(up)/Cs ≈ 10%, independent of CS and λ. Note also that the number n of identical high contrast spurious Fourier images within the depth of field Δz = (αu)-1 (α beam divergence) decreases with increasing high voltage, since n = 2Δz/Δff = θ/α = λu/α (θ the scattering angle). Thus image matching becomes easier in semiconductors at higher voltage because there are fewer high contrast identical images in any focal series.

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Direct-methods structure determination of a trypanosome RNA-editing substrate fragment with translational pseudosymmetry

Acta Crystallographica Section D Structural Biology ◽

10.1107/s2059798316001224 ◽

2016 ◽

Vol 72 (4) ◽

pp. 477-487 ◽

Cited By ~ 3

Author(s):

Blaine H. M. Mooers

Keyword(s):

Crystal Structure ◽

Random Phase ◽

Direct Methods ◽

Automated Detection ◽

Asymmetric Unit ◽

Resolution Limit ◽

Data Resolution ◽

Rna Hairpin ◽

Correct Structure

Using direct methods starting from random phases, the crystal structure of a 32-base-pair RNA (675 non-H RNA atoms in the asymmetric unit) was determined using only the native diffraction data (resolution limit 1.05 Å) and the computer programSIR2014. The almost three helical turns of the RNA in the asymmetric unit introduced partial or imperfect translational pseudosymmetry (TPS) that modulated the intensities when averaged by thelMiller indices but still escaped automated detection. Almost six times as many random phase sets had to be tested on average to reach a correct structure compared with a similar-sized RNA hairpin (27 nucleotides, 580 non-H RNA atoms) without TPS. More sensitive methods are needed for the automated detection of partial TPS.

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Relocating Features Across Microanalytical Instrumentation

Microscopy and Microanalysis ◽

10.1017/s1431927600022741 ◽

1998 ◽

Vol 4 (S2) ◽

pp. 524-525

Author(s):

William H. Powers ◽

Frederick H. Schamber

Keyword(s):

Compositional Analysis ◽

Optical Microscope ◽

The Other ◽

Depth Of Field ◽

Large Depth ◽

Contrast Mechanisms ◽

Backscattered Electron ◽

The Difference ◽

Distinguishing Features

The ability to investigate a particular feature or region of a specimen using differing kinds of analytic instrumentation can be a very valuable technique. For example, an object viewed under an optical microscope provides color, polarization, and reflectance information which is not available via SEM viewing. Similarly, the SEM with its large depth of field and ability to utilize secondary and backscattered electron contrast mechanisms, and its capability to perform compositional analysis via EDS, provides information which the optical microscope cannot. Thus, the information provided by the two instruments is complementary.Desireable though it may be to view the same feature of a specimen via differing instruments, this is often difficult to execute in practice. The difference in contrast mechanisms which makes the excercise valuable may also make it difficult to relate the distinguishing features viewed under one instrument with those viewed in the other. Further, the issue of scale creates a large complication in many practical instances.

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Kernel-phases for high-contrast detection beyond the resolution limit

10.1117/12.894319 ◽

2011 ◽

Cited By ~ 5

Author(s):

Frantz Martinache

Keyword(s):

High Contrast ◽

Resolution Limit ◽

Contrast Detection

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The Crystal Structure of Eu11Sb10

Zeitschrift für Naturforschung B ◽

10.1515/znb-1979-0909 ◽

1979 ◽

Vol 34 (9) ◽

pp. 1213-1217 ◽

Cited By ~ 10

Author(s):

R. Schmelczer ◽

D. Schwarzenbach ◽

F. Hulliger

Keyword(s):

Crystal Structure ◽

Structure Type ◽

Thermal Parameters ◽

The Other ◽

Interatomic Distances ◽

Space Group ◽

Type Space ◽

True Structure

Abstract Eu11Sb10 crystallizes in the tetragonal Ho11Ge10 structure type, space group I4/mmm, with a = 12.325(2), c = 18.024(4) Å; Z = 4. Large thermal parameters of certain atoms and unusual interatomic distances might suggest that the true structure is slightly distorted, but attempts to desymmetrize the structure were unsuccessful. Analogous anomalies occur in most of the other representatives of the Ho11Ge10 type. Eu11Sb10 appears to be metallic. It contains divalent Eu and is antiferromagnetic below TN ≈ 5 K. Other new representatives of the Ho11Ge10 structure type are Sr11Sb10, Sr11Bi10 and Ba11Sb10.

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Improvement of Stem Images by Means of Elimination of Influence of Statistical Fluctuation in Incident Electrons

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100179671 ◽

1990 ◽

Vol 48 (1) ◽

pp. 184-185

Author(s):

Hibino Michio ◽

Takahiro Suzuki ◽

Takaaki Hanai ◽

Susumu Maruse

Keyword(s):

Standard Deviation ◽

Poisson Distribution ◽

New Method ◽

Statistical Fluctuation ◽

Limit Set ◽

Resolution Limit ◽

Two Stages ◽

Polynomial Distribution ◽

Statistical Noise

The noise, due to statistical fluctuation of signal electrons, determines the ultimate resolution limit of imaging and analysis. The signal electrons (N) for each pixel are formed through two stages of statistical processes; Poisson distribution for the number of incident electrons (No) and polynomial distribution for forming signal electrons. The signal electrons therefore obey the Poisson distribution of average nOp (no:average of incident electrons, p:probability of forming signal electrons) and of standard deviation . In order to improve the resolution limit set by the statistical noise of signal electrons, a new method of normalization is proposed for STEM.In STEM imaging of thin specimens, the total transmitted electrons correspond to the incident electrons. It is therefore possible to know the incident electrons by summing up all the transmitted electrons and to normalize the signal electrons by incident electrons, for eliminating the influence of fluctuations of incident electrons. The average E and the standard deviation σ of normalized signal electrons are expressed by p and , respectively, for no> > 1 , indicating that E/σ value is improved by a factor of .

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Crystal structure of the monoclinic perovskite Sr3.94Ca1.31Bi2.70O12

Powder Diffraction ◽

10.1017/s0885715600011131 ◽

2000 ◽

Vol 15 (4) ◽

pp. 227-233 ◽

Cited By ~ 3

Author(s):

W. Wong-Ng ◽

J. A. Kaduk ◽

Q. Huang ◽

R. S. Roth

Keyword(s):

Crystal Structure ◽

Low Temperature ◽

Powder Diffraction ◽

Diffraction Data ◽

Perovskite Phase ◽

Ordered Structure ◽

Powder Diffraction Data ◽

Structural Refinement ◽

X Ray ◽

True Structure

The crystal structure of the low-temperature oxidized form of Sr49.5Ca16.5Bi34O151 has been determined using a combination of neutron, synchrotron, and laboratory X-ray powder diffraction data. The structure is pseudo-orthorhombic; systematic absences and successful refinement indicated the true structure to be monoclinic, with space group P2l/n. Structural refinement using only neutron powder data yielded the lattice parameters a=8.38 898(29) Å, b=5.99 334(21) Å, c=5.89 586(20) Å, β=89.997(8)°, and V=296.43(3) Å3. This compound is a distorted perovskite phase [described in the perovskite ABO3 formula as Sr(Bi0.7Ca0.3)O3] with ordering of the M-site cations, resulting in the formula A2MM′O6. In this ordered structure, the A sites are solely occupied by Sr, the M sites mainly by Bi, while on the M′ sites Bi and Ca are distributed in an approximate ratio of 2:3. The MO6 and M′O6 octahedra share corners, and are tilted with respect to the neighboring layers with an angle of ∼15° around all three axes. The tilt system symbol is a+a−a− according to Glazer notation. All Bi ions are in the 5+ oxidation state.

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The derivation of trial structures. I. Analytical methods for direct phase determination

Crystal Structure Analysis ◽

10.1093/oso/9780199576340.003.0017 ◽

2010 ◽

Author(s):

Jenny Pickworth Glusker ◽

Kenneth N. Trueblood

Keyword(s):

Crystal Structure ◽

Structure Determination ◽

Direct Methods ◽

Analytical Techniques ◽

Patterson Function ◽

Structure Factors ◽

Term Trial ◽

Trial Structure ◽

Electron Density Map ◽

True Structure

As indicated at the start of Chapter 4, after the diffraction pattern has been recorded and measured, the next stage in a crystal structure determination is solving the structure—that is, finding a suitable “trial structure” that contains approximate positions for most of the atoms in the unit cell of known dimensions and space group. The term “trial structure” implies that the structure that has been found is only an approximation to the correct or “true” structure, while “suitable” implies that the trial structure is close enough to the true structure that it can be smoothly refined to give a good fit to the experimental data. Methods for finding suitable trial structures form the subject of this chapter and the next. In the early days of structure determination, trial and error methods were, of necessity, almost the only available way of solving structures. Structure factors for the suggested “trial structure” were calculated and compared with those that had been observed. When more productive methods for obtaining trial structures—the “Patterson function” and “direct methods”—were introduced, the manner of solving a crystal structure changed dramatically for the better. We begin with a discussion of so-called “direct methods.” These are analytical techniques for deriving an approximate set of phases from which a first approximation to the electron-density map can be calculated. Interpretation of this map may then give a suitable trial structure. Previous to direct methods, all phases were calculated (as described in Chapter 5) from a proposed trial structure. The search for other methods that did not require a trial structure led to these phaseprobability methods, that is, direct methods. A direct solution to the phase problem by algebraic methods began in the 1920s (Ott, 1927; Banerjee, 1933; Avrami, 1938) and progressed with work on inequalities by David Harker and John Kasper (Harker and Kasper, 1948). The latter authors used inequality relationships put forward by Augustin Louis Cauchy and Karl Hermann Amandus Schwarz that led to relations between the magnitudes of some structure factors.

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The crystal structure of MoO2(O2)(H2O)·H2O

Powder Diffraction ◽

10.1017/s0885715619000095 ◽

2019 ◽

Vol 34 (1) ◽

pp. 44-49 ◽

Cited By ~ 1

Author(s):

Joel W. Reid ◽

James A. Kaduk ◽

Lidia Matei

Keyword(s):

Crystal Structure ◽

Software Package ◽

Density Functional ◽

Parallel Tempering ◽

Powder Diffraction Data ◽

Functional Theory ◽

Synchrotron Powder Diffraction ◽

Monoclinic Lattice ◽

Coordinated Water ◽

True Structure

The crystal structure of MoO2(O2)(H2O)·H2O has been solved using parallel tempering with the FOX software package and refined using synchrotron powder diffraction data obtained from beamline 08B1-1 at the Canadian Light Source. Rietveld refinement, performed with the software package GSAS, yielded monoclinic lattice parameters of a = 17.3355(5) Å, b = 3.83342(10) Å, c = 6.55760(18) Å, and β = 91.2114(27)° (Z = 4, space group I2/m). The structure is composed of double zigzag molybdate chains running parallel to the b-axis. The Rietveld refined structure was compared with density functional theory (DFT) calculations performed with CRYSTAL14, and shows comparable agreement with two DFT optimized structures of similar energy, which differ by the location of the molybdate coordinated water molecule. The true structure is likely a disordered combination of the two DFT optimized structures.

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