Nanocluster Model of Intermetallic Compounds with Giant Unit Cells: β, β′-Mg2Al3Polymorphs

2010 ◽  
Vol 49 (4) ◽  
pp. 1811-1818 ◽  
Author(s):  
Vladislav A. Blatov ◽  
Gregory D. Ilyushin ◽  
Davide M. Proserpio
Keyword(s):  
Intermetallic Compounds ◽  
Unit Cells ◽  
Nanocluster Model

Defects in Intermetallic Compounds: How Do They Differ From Those in Ordinary Metals?

MRS Bulletin ◽  
1995 ◽  
Vol 20 (7) ◽  
pp. 29-36 ◽  
Author(s):  
Y.Q. Sun
Keyword(s):  
Intermetallic Compounds ◽  
Elastic Stress ◽  
Three Dimensional ◽  
Chemical Properties ◽  
Three Dimensions ◽  
Superlattice Structure ◽  
Unit Cells ◽  
Line Defects ◽  
Physical And Chemical ◽  
And Chemical Properties

Just as in ordinary metals, defects in intermetallic compounds fall into three basic categories: point defects (vacancies, substitutional and interstitial atoms), line defects (dislocations), and planar defects (stacking faults, interfaces, grain boundaries). Also like ordinary metals, many important physical and chemical properties of intermetallic compounds are governed by the presence of these defects and the effects on them from temperature, composition, chemical environment, elastic stress state, and so on.What ultimately distinguishes an intermetallic compound from ordinary metals is its superlattice crystal structure. A two-dimensional analogue of the actual three-dimensional superlattice structure is shown in Figure 1a where the superlattice (unit cell marked by full lines) is made up of two identical sublat-tices (unit cells marked by dotted lines). A property of the sublattice is that it is exclusively occupied by one atom species, and accordingly sublattices are named after the atoms that occupy them, for example, the A and B sublattices in Figure 1a. In three dimensions, a super-lattice may consist of several sublattices. For example, the L12 superlattice of Ni3Al consists of four interpenetrating cubic sublattices, one occupied by Al atoms (Al sublattice), the other three by Ni atoms (Ni sublattices). When the sub-lattices are occupied exclusively by their designated atoms, the crystal is said to be fully ordered. The crystal will be partially ordered if a certain fraction of the sublattice sites is taken up by atoms that would otherwise sit at other sublattices; this fraction is used to describe the degree of long-range order.

Notizen: Ternäre Gamma-Messing Phasen in den Systemen Calcium-MIB(IIB)-Quecksilber / New Ternary Gamma-Brass Phases in the Systems Ca-MIB(IIB)-Hg

1980 ◽  
Vol 35 (12) ◽  
pp. 1594-1595 ◽  
Author(s):  
Z. Ban ◽  
M. Pušelj
Keyword(s):  
Intermetallic Compounds ◽  
Powder Diffraction ◽  
Structure Type ◽  
Density Measurements ◽  
X Ray ◽  
Unit Cells ◽  
Diffraction Patterns ◽  
Ternary Intermetallic Compounds

Abstract A series of new ternary intermetallic compounds of the general composition Ca16M18IB(IIB)-Hg18 (M = Zn, Cd, Hg, Cu, Ag and Au) has been identified. The X-ray powder diffraction patterns were indexed on a basis of primitive cubic unit cells.From the X-ray data and density measurements it is concluded that these phases belong to the partially disordered (MIB(IIB) and Hg) gamma-brass structure type D83.

Structures of Zr2Co11 and HfCo7 intermetallic compounds

1991 ◽  
Vol 24 (6) ◽  
pp. 1023-1026 ◽  
Author(s):  
B. G. Demczyk ◽  
S. F. Cheng
Keyword(s):  
Crystal Structure ◽  
Intermetallic Compounds ◽  
Convergent Beam Electron Diffraction ◽  
Orthorhombic Crystal ◽  
Orthorhombic Crystal Structure ◽  
Beam Electron ◽  
Long Period ◽  
Transmission Electron ◽  
Unit Cells ◽  
Convergent Beam

The crystal structures of Zr2CO11 and HfCo7 intermetallic compounds were examined by transmission electron microscopy using both selected-area and convergent-beam electron diffraction. Results show that both have an orthorhombic crystal structure, space group Pcna. The unit cells of both compounds appear to be comprised of two long-period superlattices in antiphase relation to one another along [001].

Measurement and Reduction of Radiation Damage in Frozen Hydrated Crystalline Specimens

1978 ◽  
Vol 36 (3) ◽  
pp. 70-77 ◽  
Author(s):  
R.M. Glaeser ◽  
S.B. Hayward
Keyword(s):  
Diffraction Pattern ◽  
Structural Information ◽  
Structural Disorder ◽  
Structural Features ◽  
Biological Macromolecules ◽  
Diffraction Intensity ◽  
Object Structure ◽  
Specimen Material ◽  
Unit Cells ◽  
Identical Unit

Highly ordered or crystalline biological macromolecules become severely damaged and structurally disordered after a brief electron exposure. Evidence that damage and structural disorder are occurring is clearly given by the fading and eventual disappearance of the specimen's electron diffraction pattern. The fading and disappearance of sharp diffraction spots implies a corresponding disappearance of periodic structural features in the specimen. By the same token, there is a oneto- one correspondence between the disappearance of the crystalline diffraction pattern and the disappearance of reproducible structural information that can be observed in the images of identical unit cells of the object structure. The electron exposures that result in a significant decrease in the diffraction intensity will depend somewhat upon the resolution (Bragg spacing) involved, and can vary considerably with the chemical makeup and composition of the specimen material.

Electron microscopy of niobium and tantalum hydrides

1978 ◽  
Vol 36 (1) ◽  
pp. 644-645
Author(s):  
T. Schober
Keyword(s):  
Electron Microscopy ◽  
Heat Storage ◽  
Measurement Techniques ◽  
Metal Hydrides ◽  
Detailed Understanding ◽  
Storage Devices ◽  
Unit Cells ◽  
Endothermic Reactions ◽  
Hydrogen Reactions ◽  
New Phase

Nb, Ta and V are prototype substances for the study of the endothermic reactions of H with metals. Such metal-hydrogen reactions have gained increased importance due to the application of metal-hydrides in hydrogen- und heat storage devices. Electron microscopy and diffraction were demonstrated to be excellent methods in the study of hydride morphologies and structures (1). - Figures 1 and 2 show the NbH and TaH phase diagrams (2,3,4). EM techniques have contributed substantially to the elucidation of the structures and domain configurations of phases β, ζ and ε (1,4). Precision length measurement techniques of distances in reciprocal space (5) recently led to a detailed understanding of the distortions of the unit cells of phases ζ and ε (4). In the same work (4) the existence of the new phase η was shown. It is stable near -68 °C. The sequence of transitions is thus below 70 %.

Crystal Structure Analysis of Cu-Zr Alloys by Convergent Beam Diffraction

1981 ◽  
Vol 39 ◽  
pp. 354-355
Author(s):  
A. F. Marshall ◽  
J. W. Steeds ◽  
D. Bouchet ◽  
S. L. Shinde ◽  
R. G. Walmsley
Keyword(s):  
Crystal Structure ◽  
Point Group ◽  
Intermetallic Phase ◽  
Three Dimensional ◽  
Crystal Structure Analysis ◽  
Ion Milling ◽  
Composition And Structure ◽  
Unit Cells ◽  
Sputter Deposited ◽  
Convergent Beam

Convergent beam electron diffraction is a powerful technique for determining the crystal structure of a material in TEM. In this paper we have applied it to the study of the intermetallic phases in the Cu-rich end of the Cu-Zr system. These phases are highly ordered. Their composition and structure has been previously studied by microprobe and x-ray diffraction with sometimes conflicting results.The crystalline phases were obtained by annealing amorphous sputter-deposited Cu-Zr. Specimens were thinned for TEM by ion milling and observed in a Philips EM 400. Due to the large unit cells involved, a small convergence angle of diffraction was used; however, the three-dimensional lattice and symmetry information of convergent beam microdiffraction patterns is still present. The results are as follows:1) 21 at% Zr in Cu: annealed at 500°C for 5 hours. An intermetallic phase, Cu3.6Zr (21.7% Zr), space group P6/m has been proposed near this composition (2). The major phase of our annealed material was hexagonal with a point group determined as 6/m.

Computer processing of high-resolution images of periodic specimens

1986 ◽  
Vol 44 ◽  
pp. 2-5
Author(s):  
W. Chiu ◽  
M.F. Schmid ◽  
T.-W. Jeng
Keyword(s):  
High Resolution ◽  
Fourier Transforms ◽  
Complex Structure ◽  
Distribution Functions ◽  
Multiple Image ◽  
Cryo Electron Microscopy ◽  
Structure Factors ◽  
Unit Cells ◽  
High Resolution Images ◽  
Phase Probability Distribution

Cryo-electron microscopy has been developed to the point where one can image thin protein crystals to 3.5 Å resolution. In our study of the crotoxin complex crystal, we can confirm this structural resolution from optical diffractograms of the low dose images. To retrieve high resolution phases from images, we have to include as many unit cells as possible in order to detect the weak signals in the Fourier transforms of the image. Hayward and Stroud proposed to superimpose multiple image areas by combining phase probability distribution functions for each reflection. The reliability of their phase determination was evaluated in terms of a crystallographic “figure of merit”. Grant and co-workers used a different procedure to enhance the signals from multiple image areas by vector summation of the complex structure factors in reciprocal space.

Correlation averaging of two-dimensional crystals: basic strategy and refinements

1986 ◽  
Vol 44 ◽  
pp. 136-139
Author(s):  
W. Baumeister ◽  
R. Rachel ◽  
R. Guckenberger ◽  
R. Hegerl
Keyword(s):  
Low Dose ◽  
Signal To Noise Ratio ◽  
Real Space ◽  
Fourier Domain ◽  
Two Dimensional ◽  
Signal To Noise ◽  
Unit Cells ◽  
Lateral Displacements ◽  
Established Technique ◽  
Crystalline Array

IntroductionCorrelation averaging (CAV) is meanwhile an established technique in image processing of two-dimensional crystals /1,2/. The basic idea is to detect the real positions of unit cells in a crystalline array by means of correlation functions and to average them by real space superposition of the aligned motifs. The signal-to-noise ratio improves in proportion to the number of motifs included in the average. Unlike filtering in the Fourier domain, CAV corrects for lateral displacements of the unit cells; thus it avoids the loss of resolution entailed by these distortions in the conventional approach. Here we report on some variants of the method, aimed at retrieving a maximum of information from images with very low signal-to-noise ratios (low dose microscopy of unstained or lightly stained specimens) while keeping the procedure economical.

TEM observations of surface-step and hole-edge faceting in acid-etched and annealed MgO thin foils

1993 ◽  
Vol 51 ◽  
pp. 1134-1135
Author(s):  
Simon King ◽  
C. Barry Carter
Keyword(s):  
Marked Contrast ◽  
Aqua Regia ◽  
Surface Step ◽  
Smoke Particle ◽  
Hole Edge ◽  
Thin Foils ◽  
Surface Steps ◽  
Unit Cells ◽  
Reflection Electron Microscopy ◽  
Similar Preparation

Surface-steps formed during the cleavage of MgO on {100} planes, the smaller steps of which may be of atomic height, have been observed in Reflection-Electron Microscopy investigations to be accurately aligned along <001> directions. Steps of atomic height also have been identified on MgO smoke-particle platelets; these steps may be curved or straight, with the straight steps showing evidence for faceting along <001>. Reference also is made to faceting along <011> and <012> directions. Straight steps ∼2 unit cells high, with edges along <100> also have been imaged by High-Resolution Profile-Imaging at the peripheries of MgO smoke microcubes. After etching in aqua-regia and annealing in air, however, high densities of “large” steps several unit cells high, as well as numerous holes, are formed. It is faceting in these foils that is reported here.As can be seen in fig 1, obvious faceting of the surface-step traces is extremely rare in these foils, in marked contrast to substrates such as LaAlO3 and SrTiO3, on which surface-step traces facet readily after a similar preparation treatment.

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