Atomic site and species determinations using channeling and related effects in analytical electron microscopy

doi:10.1016/0304-3991(88)90306-3

Ultramicroscopy

Volume 24, Issues 2–3, 1988, Pages 125-141

https://doi.org/10.1016/0304-3991(88)90306-3 Get rights and content

Abstract

The formulation, development and applications of a novel crystallographic technique for specific site occupation determinations using the channeling or Borrmann effect in electron diffraction is reviewed. This technique is based on the effect of incident beam orientations on the intensities of either the characteristic X-ray emission or the characteristic energy-loss edges. The formulation of the technique under planar-channeling conditions for simple layered structures (ALCHEMI — atom location by channeling-enhanced microanalysis) and a general formulation for non-layered structures, along with the relevant theory, are reviewed in detail. In the case of characteristic energy-loss edges, in addition to being a function of the diffraction of the incident beam (channeling), the intensities are also a function of the diffraction of the outgoing inelastically scattered fast electron (blocking) and the scattering angle. A judicious choice of these parameters can provide additional information, such as the specific site valence of a particular atomic species. In general, this technique can distinguish neighbors in the periodic table; involves no adjustable parameters, external standards or special specimen preparations; is applicable to trace element concentrations (0.2 – 0.3 wt% or 10²⁵ atoms/m³); is very accurate (3 – 10% error in site occupancy determinations, depending on the formulation used); and can be routinely applied at very high spatial resolutions (10 – 40 nm). Two crucial assumptions are made: the inelastic scattering events are assumed to be highly localized and the impurities/additions are assumed to be distributed uniformly with depth in the specimen. These and other assumptions, limitations and possible improvements or extensions of the technique are discussed, but throughout the review, the emphasis is directed to practical considerations.

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Cited by (27)

Site occupancy of alloying elements in the L1<inf>2</inf> structure determined by channeling enhanced microanalysis in γ/γ’ Co-9Al-9W-2X alloys
2019, Acta Materialia
Citation Excerpt :
As increasing the convergence angle, the ionization cross-sections slightly decrease and the channeling intensity of the normalized Al-K cross-section is also slightly reduced. This stems from the fact that the channeling condition is fulfilled for a planar incident electron wave and thus a bigger convergence angle causes a higher deviation of the incident beam from such an ideal planar wave character, resulting in a reduced channeling intensity [37,38]. On increasing the electron voltage (convergence angle kept at 4 mrad and specimen thickness at 100 nm), the normalized cross-section profiles of the Al-K line change obviously, especially for scattering conditions between the (00–2) and the (002) reflections.
Knowledge about the sublattice site preference of alloying elements in the L1₂-γ′ phase of novel Co-base superalloys is a necessary pre-requisite to understand their influence on the properties of the alloys in general and the γ′ phase in particular. In the present study, the atomic site occupancy of the alloying elements in the L1₂-γ′ structure in Co-9Al-9W-2X quaternary alloys after long-term annealing at 900 °C for 5000 h was determined using the atom location by channeling enhanced microanalysis (ALCHEMI) technique in combination with energy-dispersive X-ray spectroscopy (EDX) composition analysis in a transmission electron microscope (TEM). The experimental ALCHEMI data were evaluated by comparing them with those calculated by the program ‘Inelastic Cross Section Calculator’ (ICSC). The results show that Co mainly occupies one sublattice site and Al/W are located at the other sublattice site in the L1₂ unit cell in the ternary alloy. The additional elements Ti, V, Mo and Ta which partition strongly to the γ′ phase tend to occupy the Al/W sublattice site, and Cr which partitions more to the γ phase also favors the Al/W sublattice site, while Ni weakly partitions into the γ′ phase and favors the Co sublattice site. The results of this study can provide evidence to the predictions on the site preference in literature based on the phase composition or on theoretical studies.
Determining the locations of chemical species in ordered compounds: ALCHEMI
2003, Advances in Imaging and Electron Physics
o
1. 1.
  Eighty percent of the results (in terms of detail) come very easily; the other 20% (exact delocalization correction, exact OTL length) require far more effort.
2. 2.
  There are two ALCHEMI analyses, for dilute and for concentrated solutions. For the concentrated solution analysis, the chemical composition of the compound is required plus one independent (i.e., non-ALCHEMI) assumption or measurement or calculation.
3. 3.
  Axial ALCHEMI gives larger variations in channeling than does planar ALCHEMI, but is more prey to delocalization.
4. 4.
  Sparsely occupied interstitial and chemically indistinguishable sublattices require a priori calculations of the electron distribution.
5. 5.
  Statistics are improved by plotting the ALCHEMI results vs. the strength of channeling, in some fashion.
6. 6.
  Either delocalization should be avoided as much as possible, or it should be corrected for by a full calculation.
7. 7.
  The optimum conditions for systematic ALCHEMI are as outlined in Section II.E.
8. 8.
  To compare compounds with different chemical compositions, use A_50-x/2B_50-x/2C_x or a series including this and/or deduce A-B-C bond enthalpies.
9. 9.
  Ordering is only sensitive to host atom stoichiometry when ΔH^A-C/ΔH^A-B ~ ΔH^B-C/ΔH^A-B.
10. 10.
  There is a paucity of results for oxidation state occupancy, for functional materials (magnetic and superconducting) and of detailed results (concentratedsolution ALCHEMI) for given compounds over ranges of compositionand temperature.
11. 11.
  Bragg-Williams calculations appear to be capable of predicting most
Cation disordering in magnesium aluminate spinel crystals induced by electron or ion irradiation
2000, Journal of Nuclear Materials
Structural changes in magnesium aluminate spinel (MgO · nAl₂O₃) single crystals, which were irradiated with 900 keV electrons or 1 MeV Ne⁺ ions at 873 K, were examined by electron channeling enhanced X-ray microanalysis. Unirradiated MgO · Al₂O₃ has a tendency to form the normal spinel configuration, where Mg²⁺ ions and Al³⁺ ions occupy mainly the tetrahedral and the octahedral sites, respectively. Electron irradiation induces simple cation disordering between the tetrahedral sites and the octahedral sites in MgO · Al₂O₃. In addition to cation disordering, slight evacuation of cations from the tetrahedral sites to the octahedral sites occurs in a peak-damaged area in MgO · Al₂O₃ irradiated with Ne⁺ ions. In contrast, cation disordering is suppressed in MgO · 2.4Al₂O₃ irradiated with electrons. The structural vacancies, present in the non-stoichiometric compound, appear to be effective in promoting irradiation damage recovery through interstitial–vacancy recombination.
Correction terms and approximations for atom location by channelling enhanced microanalysis
1999, Ultramicroscopy
A theory for X-ray emission from crystals, when subjected to electron beam irradiation under dynamical electron diffraction conditions, is used to derive correction terms which account for differences in measured responses from atoms due to variations in delocalization and X-ray absorption. The (e,2e) interaction kernel, involving ionization of target atoms within the lattice, is modelled using Hartree–Fock atomic bound-state wavefunctions. Incoherent channelling patterns are calculated for Al K-shell as well as Co K- and L-shell emissions from beta phase AlCo for a 250 keV electron beam and compared with experiment. This theory enables a correction factor G^s to be defined for each orientation s, facilitating an exact implementation of statistical ALCHEMI (atom location by channelling enhanced microanalysis), referred to as Model A. Various levels of approximation are then applied to this exact model and compared with previous implementations of ALCHEMI analysis. In a first approximation (Model B), an averaged correction factor G is used to calculate an offset constant C for the overall fit between dopant and host atom responses, and G is used to extract relative partitioning from the fitted parameters. The next approximation, Model C, allows the offset constant to be determined from the fitting procedure itself. The last Model D has no correction terms in the fitting procedures. Experimental data were found to have sufficient systematic error that it was not useful to assess accurately the differences between these models. Thus pseudodata were used to investigate both precision and goodness of fit parameter for these models for ALCHEMI. The effect of increasing X-ray detection times (and hence decreasing relative levels of statistical noise) was considered. The theory also yields a measure of ionization delocalization from first principles, and we find the mean impact parameter for Co L-shell ionization to be about twice that derived from the previous semiclassical estimates.
Site-distributions of Fe alloying additions to B2-ordered NiAl
1999, Intermetallics
The site-distributions of Fe alloying additions to B2-ordered NiAl alloys have been measured by ALCHEMI ( $a$ tom $l$ ocation by $ch$ anneling- $e$ nhanced $m$ icroanalysis). Twelve alloy compositions were examined, of stoichiometries Ni_50−xAl₅₀Fe_x (Ni-deficient), Ni₅₀Al_50−xFe_x (Al-deficient), and Ni_50−x/2Al_50−x/2Fe_x (intermediate), with four different alloying levels, x=0.25, 2, 5, and 10. The data indicate that Fe tends to act as a buffer between the two sublattices, preferentially occupying the site of the stoichiometrically deficient host element. However, the Fe alloying addition was found to partition between the two sublattices to some extent in all alloys, so that ‘Ni’-site vacancies or Ni anti-site defects are required to maintain the site balance. The observed trends in the site-distributions suggest that the range of compositions reported by Darolia et al. (Darolia R, Lahrman DF, Field RD. Scripta Metall Mater 1992;26:1007). to exhibit enhanced ductility may coincide with an inversion in the site-substitution behavior, where more Ni than Fe occupies the aluminum sublattice.
Alchemi study of site distributions of 3d-transition metals in B2-ordered iron aluminides
1997, Acta Materialia
ALCHEMI has been used to determine the site distributions of the 3d-transition metals Me = {Ti, V, Cr, Mn, Co, Ni, Cu} in B2-ordered iron aluminides of stoichiometry Fe₅₀Al₄₅Me₅. These alloying additions exhibited an increasing ‘Fe’-site preference with increasing atomic number. Elements Co, Ni and Cu were found to occupy the ‘Fe’ site exclusively, displacing 10% of the Fe host element to the ‘Al’ anti-site. None of the alloying elements exhibited an exclusive ‘Al’-site preference. The element Ti exhibited the largest ‘Al’-site occupancy, with ∼85% of the alloying element occupying the ‘Al’ site. The residual ‘Fe’-site occupancy (15%) can be attributed to the kinetics of the site-equilibration mechanism.

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Atomic site and species determinations using channeling and related effects in analytical electron microscopy

Abstract

Phys. Letters

Nucl. Instr. Methods

Ultramicroscopy

J. Solid State Chem.

Ultramicroscopy

Ultramicroscopy

Ultramicroscopy

Ultramicroscopy

Ultramicroscopy

Ultramicroscopy

Rev. Mod. Phys.

Radiation Effects

Introduction to Solid State Physics

J. Phys. Soc. Japan

Phil. Mag.

Physik. Z.

Acta Cryst.

Phil. Mag.

Phil. Mag.

Phil. Mag.

Z. Naturforsch.

Acta Cryst.

Phys. Rev. Letters

J. Microscopy

J. Microscopy

Acta Cryst.

Phys. Rev. Letters