Zone-axis ALCHEMI for the rapid assessment of site occupancies in garnets

1986 ◽  
Vol 44 ◽  
pp. 706-707
Author(s):  
M.T. Otten ◽  
P.R. Buseck
Keyword(s):  
Single Crystals ◽  
Rapid Assessment ◽  
Site Occupancy ◽  
Zone Axis ◽  
Synthetic Compounds ◽  
X Ray ◽  
Large Group ◽  
Crystallographic Planes

ALCHEMI (Atom Location by CHannelling-Enhanced Microanalysis) is a TEM technique for determining site occupancies in single crystals. The method uses the channelling of incident electrons along specific crystallographic planes. This channelling results in enhanced x-ray emission from the atoms on those planes, thereby providing the required site-occupancy information. ALCHEMI has been applied with success to spinel, olivine and feldspar. For the garnets, which form a large group of important minerals and synthetic compounds, the channelling effect is weaker, and significant results are more difficult to obtain. It was found, however, that the channelling effect is pronounced for low-index zone-axis orientations, yielding a method for assessing site occupancies that is rapid and easy to perform.

Overvoltage and diffusion through iron and palladium

1959 ◽  
Vol 253 (1272) ◽  
pp. 6-15 ◽  
Keyword(s):  
Crystal Lattice ◽  
Single Crystals ◽  
Molecular Hydrogen ◽  
X Ray Diffraction ◽  
Palladium Hydride ◽  
X Ray ◽  
Crystallographic Planes ◽  
And Diffusion ◽  
Diffusion Of Hydrogen ◽  
Crystal Boundaries

Observations are recorded on X-ray spectra diffracted from cathodes of iron and palladium during actual discharge and diffusion of hydrogen. For iron, no expansion of the crystal lattice greater than 0.0002 Å could be observed during such diffusion, and the broadening observed of X-ray diffraction spectra indicated no preferred crystallographic planes for the evolution of molecular hydrogen. Blisters were formed across the faces of single crystals of iron, when made a cathode, indicating that diffusion of hydrogen occurs through the crystal grains rather than along crystal boundaries. For palladium, an expansion was observed during actual electrolysis to an extent of 0.0170 + 0.0002 Å. On ceasing electrolysis, there was reversion to a less-expanded lattice of palladium hydride. Broadening of the reflexions observed during electrolysis indicated no preferred planes for evolution of hydrogen. The significance of these findings is discussed for mechanisms of overvoltage and of diffusion of hydrogen through metals.

Growth and Structure of Zn1-xMnxCr2Se4 Single Crystals

2007 ◽  
Vol 130 ◽  
pp. 81-84 ◽  
Author(s):  
Izabela Jendrzejewska ◽  
Alicja Waśkowska ◽  
Elżbieta Augustyn
Keyword(s):  
Chemical Composition ◽  
Single Crystals ◽  
Cation Distribution ◽  
Chemical Vapour ◽  
Site Occupancy ◽  
Vapour Transport ◽  
X Ray Diffraction ◽  
X Ray ◽  
Chemical Vapour Transport ◽  
Tetrahedral Sites

The single crystals of ZnCr2Se4 spinel doped with manganese were prepared by the chemical vapour transport in closed silica tubes using stoichiometric contents of ZnSe, MnSe and CrCl3 as the transporting agent. Chemical composition of four single crystals of Zn-Mn-Cr-Se system with different concentration of manganese was analyzed and confirmed by X-ray diffraction study. The structure refinements, taking into account site occupancy factors, indicated that Mn ion shared tetrahedral sites with Zn ion. Thus, the formula describing cation distribution in the system is: Zn1-xMnxCr2Se4. For all x values equal to 0.12, 0.13, 0.18 and 0.24, the observed symmetry was cubic, space group Fd 3m .

Site occupations in L12 ordered alloys by axial electron channeling microanalysis

1986 ◽  
Vol 44 ◽  
pp. 704-705
Author(s):  
J. Bentley
Keyword(s):  
Lattice Site ◽  
Site Occupancy ◽  
Zone Axis ◽  
Site Location ◽  
X Ray Diffraction ◽  
Alloy Development ◽  
X Ray ◽  
Ion Channeling ◽  
Ordered Alloys ◽  
Lattice Planes

Lattice site location of alloying additions to A3B-type ordered alloys with the L12 structure is important for alloy development, but ion channeling and X-ray diffraction techniques require single crystal specimens. The ALCHEMI technique is applicable to polycrystalline specimens but provides only qualitative results on the site occupancy in L12 A3B alloys. Quantitative analysis by standard planar channeling ALCHEMI is complicated by the absence of lattice planes containing only B atoms. Since certain zone axes, including <001> and <111>, contain columns of A atoms well separated from columns of B atoms, the ALCHEMI method has been extended to use zone axis channeling conditions. Further, the procedure has been simplified to use spectra recorded from two orientations - an appropriate channeling zone axis and a “random” non-channeling condition typically 2-5° from the zone. The concentration CX of element X on the B sites is given by Cx = {R(X/A) - 1}/{R(B/A) - 1}, where R(X/A) = (NX/NA)zone/(NX/NA)random, ete., and NX, NA, NB are the characteristic X-ray intensities of elements X, A, and B.

scholarly journals Slip bands on mercury single crystals

1937 ◽  
Vol 163 (912) ◽  
pp. 28-34 ◽  
Keyword(s):  
Single Crystals ◽  
Cross Section ◽  
Crystal Surface ◽  
Circular Cross Section ◽  
Critical Shear Stress ◽  
Relative Displacement ◽  
General Term ◽  
X Ray ◽  
Slip Bands ◽  
Crystallographic Planes

In the course of an experimental investigation of the factors determining the critical shear stress of mercury single crystals, it became clear that the distortion of the crystals by gliding took place in a way very different from that illustrated by the classical Polanyi model of gliding. This model represents the crystal in the form of a rod of circular cross-section, which, when stretched, separates into a number of inclined disks or glide lamellae which slide over each other. The planes defining the lamellae are crystallographic and are called glide planes. The relative displacement of the glide lamellae results in the appearance of markings on the crystal surface which are the traces of the intersection of the glide planes with the surface. These, according to the Polanyi model, are elliptical in shape, and so have been called “glide ellipses”, but in this paper the more general term “slip band” will be used. This representation of the process of distortion is in perfect accord with the observed phenomena in the majority of metal crystals. Their slip bands are perfect in shape and can be shown, by X-ray examination, to correspond closely to the traces of crystallographic planes.

An ALCHEMI Investigation of Ternary Site Occupancy in NiAl-Based Alloys

1990 ◽  
Vol 48 (2) ◽  
pp. 448-449 ◽  
Author(s):  
P.R. Munroe ◽  
I. Baker
Keyword(s):  
Lattice Site ◽  
Site Occupancy ◽  
Zone Axis ◽  
Ternary Addition ◽  
Bragg Condition ◽  
Qualitative Information ◽  
X Ray ◽  
Clear Result ◽  
Ternary Element ◽  
Ternary Additions

ALCHEMI, or atom location by channeling enhanced microanalysis, is a technique used for determining lattice site occupancies. Previously it has been applied to sublattice occupancy of trace elements in minerals and ternary additions to binary intermetallic compounds. Normally this is performed by acquiring spectra at both positive and negative deviations from the Bragg condition at a suitable plane. (Alternatively, spectra may be recorded at, and away from, a zone axis). So, for example, in a compound AB+X, NX, the integrated x-ray intensity of the element X, is ratioed to both NA and NB for x-ray spectra acquired at both +sg and -sg. If either NX/NA or NX/NB is independent of sg this infers that the ternary element occupies the A or B sublattice respectively, thus providing qualitative information. However, if the ternary addition is partitioned between the two sites, this method is unsatisfactory and comparison of the ratios of the x-ray intensities will not provide a clear result.

High-resolution structure determination by ALCHEMI

1983 ◽  
Vol 41 ◽  
pp. 178-181 ◽  
Author(s):  
Peter G. Self ◽  
Peter R. Buseck
Keyword(s):  
Site Occupancy ◽  
Real Space ◽  
Unit Cell ◽  
Bragg Angle ◽  
Electron Current ◽  
High Resolution Structure ◽  
Beam Incident ◽  
Crystallographic Planes ◽  
Electron Beam Incident ◽  
Resolution Structure

ALCHEMI (Atom Location by CHanneling Enhanced Microanalysis) enables the site occupancy of atoms in single crystals to be determined. In this article the fundamentals of the method for both EDS and EELS will be discussed. Unlike HRTEM, ALCHEMI does not place stringent resolution requirements on the microscope and, because EDS clearly distinguishes between elements of similar atomic number, it can offer some advantages over HRTEM. It does however, place certain constraints on the crystal. These constraints are: a) the sites of interest must lie on alternate crystallographic planes, b) the projected charge density on the alternate planes must be significantly different, and c) there must be at least one atomic species that lies solely on one of the planes.An electron beam incident on a crystal undergoes elastic scattering; in reciprocal space this is seen as a diffraction pattern and in real space this is a modulation of the electron current across the unit cell. When diffraction is strong (i.e., when the crystal is oriented near to the Bragg angle of a low-order reflection) the electron current at one point in the unit cell will differ significantly from that at another point.

Microdomains in BaFeO3-y (0.35 < y < 0.50)

1990 ◽  
Vol 48 (4) ◽  
pp. 780-781
Author(s):  
M. Vallet-Regí ◽  
M. Parras ◽  
J.M. González-Calbet ◽  
J.C. Grenier
Keyword(s):  
Electron Diffraction ◽  
Chemical Analysis ◽  
Monoclinic Phase ◽  
Orthorhombic Phase ◽  
Total Iron ◽  
Zone Axis ◽  
Electron Micrograph ◽  
X Ray Diffraction ◽  
X Ray ◽  
Compositional Variations

BaFeO3-y compositions (0.35<y<0.50) have been investigated by means of electron diffraction and microscopy to resolve contradictory results from powder X-ray diffraction data.The samples were obtained by annealing BaFeO2.56 for 48 h. in the temperature range from 980°C to 1050°C . Total iron and barium in the samples were determined using chemical analysis and gravimetric methods, respectively.In the BaFeO3-y system, according to the electron diffraction and microscopy results, the nonstoichiometry is accommodated in different ways as a function of the composition (y):In the domain between BaFeO2.5+δBaFeO2.54, compositional variations are accommodated through the formation of microdomains. Fig. la shows the ED pattern of the BaFeO2.52 material along thezone axis. The corresponding electron micrograph is seen in Fig. 1b. Several domains corresponding to the monoclinic BaFeO2.50 phase, intergrow with domains of the orthorhombic phase. According to that, the ED pattern of Fig. 1a, can be interpreted as formed by the superposition of three types of diffraction maxima : Very strong spots corresponding to a cubic perovskite, a set of maxima due to the superposition of three domains of the monoclinic phase along [100]m and a series of maxima corresponding to three domains corresponding to the orthorhombic phase along the [100]o.

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