Channeling-enhanced Microanalysis Research Articles

Atom Location by Channeling-Enhanced Microanalysis (ALCHEMI) Study of Structure of Complex Concentrated Alloys Derived From FeAl Iron Aluminide

Atom Location by Channeling-Enhanced Microanalysis (ALCHEMI) Study of Structure of Complex Concentrated Alloys Derived From FeAl Iron Aluminide

Automating ALCHEMI at the nano-scale using software compatible with PC-controlled transmission electron microscopy.

Atom location by channeling-enhanced microanalysis (ALCHEMI) is a technique to obtain atom-site-specific information on constituent elements in a crystalline sample by acquiring a set of core electron transition spectra while tilting the incident beam. This methodology has been extended to a more quantitative technique called high-angular-resolution electron-channeled X-ray/electron spectroscopy (HARECXS/HARECES). There is a growing demand for analyzing smaller areas, such as small particles and multilayers. However, the minimum size of a region of interest probed by the present hardware-assisted automated HARECXS/HARECES scheme is limited to no smaller than 1 µm, not only by the size of the electron probe and its convergence angle but also by the movement of the probe position associated with the beam tilt due to aberrations of the hardware system. Herein, QED (quantitative electron diffraction), a commercial plug-in working on an integrated software platform, Gatan Microscopy Suite, was modified to enable the calibration and control of the probe to resolve the aforementioned limitation. In addition, a more sophisticated scheme for QED was developed to realize the ALCHEMI method for energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy or both concurrently. This allows access to ALCHEMI and its derivative methods, automatically executed with any type of current PC-controlled commercial microscope on an area as small as 30 nm, without modifying the hardware system.

Characterization of Al-Si ordering state in an alkali feldspar using atom location by channeling-enhanced microanalysis (ALCHEMI)

Atom location by channeling-enhanced microanalysis (ALCHEMI) was used to determine the occupancy of Al atoms in the T1o site (t1o) of alkali feldspar. Building on the method demonstrated by previous research, analytical electron microscopy proves to be a viable technique for fully characterizing the Al-Si ordering state in the feldspar framework. We applied this method to orthoclase from Itrongay, Madagascar, and to its heated counterpart. Our preliminary results give 0.74 and 0.37 as 2t1 (=t1o + t1m) and t1o, respectively, for the original orthoclase, vs. 0.54 and 0.28 for heated orthoclase. Single-crystal X-ray diffraction experiments have been performed, and the results agree with our ALCHEMI measurements. This new method promises to help resolve some complex issues relating ordering paths, for example, twinning in feldspars and domain intergrowth in plagioclase.

Electron energy losses by channelling-enhanced microanalysis in a γ′-strengthened Ni-base alloy with Ti additions

The addition of Ti to a γ′-strengthened Ni-base model alloy shows a strong preference of Ti for the γ′-phase. Ti strongly influences the diffusion behaviour of constituents of the Ni 3Al (γ′) phase, although the mechanism is uncertain. Electron energy loss channelling experiments were therefore performed to determine the electronic structure in the vicinity of the Ti atom. The experimental results showed an increase in the Ti L 3/L 2 ratio when probing the Ni sublattice planes compared to the Al sublattice planes, indicating a strong charge transfer from the Ti to the Ni atoms. This finding was supported by first-principles calculations, indicating an increase in electron density in the vicinity of the Ti atom and a strengthening of the chemical bonds.

Polarity Determination for the CdxZn1-xS Nanocombs by EELS

Comb-like Cd(x)Zn(1-x)S nanostructures have been synthesized by one-step metallo-organic chemical vapor deposition (MOCVD). The microstructure and composition were characterized by analytical transmission electron microscopy (TEM). The ribbon of the comb-like structure is along [2-1-10] (the a-axis), and the nanotips are along [0001] (the c-axis). The atomic location by channeling-enhanced microanalysis (ALCHEMI) technique was used to determine the polarity of the crystal via electron energy-loss spectroscopy (EELS). The thickness dependence, delocalization and beam convergent effect are discussed here. The investigations reveal that the growth mechanism is consistent with the asymmetric growth.

ALCHEMI for coexistent Heusler and half-Heusler phases in TiNi 1.5Sn

Atomic location by channeling-enhanced microanalysis (ALCHEMI) is an effective technique to clarify the atomic configurations in multicomponent compounds. Recent development of the theory on the characteristic X-ray emission has made ALCHEMI a more reliable and expansive technique. In this revised ALCHEMI, the characteristic X-ray intensities are measured at various electron-incidence directions, and are compared with the X-ray intensities calculated from first principles. In the present work, this technique was applied to coexistent Heusler and half-Heusler phases in a thermoelectric material, TiNi 1.5Sn. The X-ray intensities calculated for both phases were significantly dependent on the fitting parameters of the atomic occupancy on each site and the sample thickness. Using the Levenberg–Marquardt least-squares method, the residual errors between the experimental and the calculated X-ray intensities were minimized to determine the fitting parameters quantitatively. In the Heusler phase, the ordered structure was observed because the atoms of each element almost occupy each site. On the other hand, in the half-Heusler phase, the “Heusler-like” structure was observed because the occupation probabilities of Ni atoms on the Ni-site and on the vacancy-site were measured as 0.49±0.01 and 0.49±0.01, respectively, whereas 1 and 0 is expected for the normal half-Huesler structure.

Incorporation of dopants into the lattice of ZnO nanoparticles to control photoactivity

Using a similar production process, a series of doped and undoped ZnO nanoparticles has been tested for photoactivity, using chemical methods, when exposed to UV radiation. Parameters such as crystallite size, dopant type and concentration were considered. It was confirmed that photoactivity increased significantly by a factor of 2 to 3 as the mean crystallite size was decreased from 100 to 20 nm. Analysis of a further series of materials doped with metal cations showed that, depending on the dopant type and concentration, photoactivity could be reduced by more than an order of magnitude. A variant of the method of atom location by channelling-enhanced microanalysis (ALCHEMI) was applied to these crystal systems to determine whether or not dopant atoms at a concentration of less than 1 at.% were incorporated on the cation sublattice site within the nanoparticles. This technique is described and discussed in some detail.

Electron channeling X-ray microanalysis for site occupation in β-FeSi 2 doped with Co

The site occupation state of Co atoms in a thermoelectric semiconductor Fe 0.97Co 0.03Si 2 of the β-FeSi 2 structure was analyzed by measuring and calculating characteristic X-ray intensities at various electron incidence directions in a transmission electron microscope (TEM). The analysis technique used was recently developed from the method for determining atomic locations by channeling-enhanced microanalysis (ALCHEMI). The calculation was based on dynamical electron diffraction and inelastic scattering theories, and the calculated intensities were compared with the measured intensities. The intensities depended on the occupation site of Co atoms and sample thickness. As a result, the occupation probabilities of Co atoms on Fe I and II sites, which were difficult to be determined by the conventional ALCHEMI technique, were shown to be 0.0333 and 0.0257, respectively.

Atom location by channelling-enhanced microanalysis and the ordering of Ti2AlNb

Atom location by channelling-enhanced microanalysis (ALCHEMI) has been applied to a Ti2AlNb compound of composition Ti–23 at.% Al–21 at.% Nb so as to deliver absolute compositions for the three sublattices. These compare well with our own neutron diffraction measurements and with previously reported neutron diffraction results. The thus-validated ALCHEMI method has been applied to a single-phase Ti2AlNb compound of composition Ti–27.5 at.% Al–23 at.% Nb annealed at three different temperatures.

Polarity determination by atomic location by channeling-enhanced microanalysis

In this letter, an alternative approach to determine the polarity of GaN thin films based on the atomic location by channeling-enhanced microanalysis technique is described. Theoretical calculations provide a straightforward criterion for polarity determination that is a major advantage of this method. At the Bragg position, the thickness-averaged incident electron intensity, and hence, electron induced characteristic x-ray yield, is higher on the N plane than on the Ga if the g vector of the diffraction beam is parallel to the Ga–N bond direction, and vice versa. Experimental results support the theoretical predictions. The possible errors in the experiments are also discussed.

How accurate is ALCHEMI for ordering measurements?

Atom location by channeling-enhanced microanalysis (ALCHEMI) promises a straightforward measurement of ordering in materials. On tilting the electron beam in a transmission electron microscope (TEM) about a superlattice Bragg position the electron channeling changes, weighting X-ray production from each of the sublattices differently. The overall X-ray signal, converted to an apparent composition via a kinematical non-channeling standardization, moves between the composition of the constitute sublattices. However, the accuracy of measurement always bothers many researchers. Results may vary from time to time, and in the worse cases, the composition outputs are nonsense. Table 1 gives an example of a series of ALCHEMIC measurements in a B2 Ti50Al42Mo8 intermetallic alloy.In general, two major sources cause errors in ALCHEMI: weak channeling and delocalization effects. The later only results in “systematic errors”, which can always be corrected or reduced by theoretical calculations. However, it is difficult to improve the error due to weak channeling.

Atomic scattering factors for K-shell and L-shell ionization by fast electrons

Atomic scattering factors have been calculated for K-shell ionization for elements in the range Z = 6 (carbon) to Z = 50 (tin) and for L-shell ionization in the range Z = 20 (calcium) to Z = 60 (neodymium). The calculations are based on relativistic Hartree-Fock wave functions for the atomic bound states and Hartree-Slater wave functions for the continuum wave functions. The results are presented in tabular form such that accurate values of the scattering factors can be obtained by cubic spline interpolation for incident electron energies between 50 and 400 keV and for scattering vectors with magnitude s = sin straight theta / lambda up to 2.5 A(-1) (2straight theta is the scattering angle and lambda the wavelength of the incident electrons). A separate parameterization of the form factors is given for 2.5 </= s </= 20 A(-1), where they are small. In addition, a simpler but less accurate parameterization of the atomic scattering factors in an exponential form has been obtained by fitting the calculated form factors in the region s </= 2.5 A(-1). The scattering factors are suitable for the calculation of ionization cross sections for use in atom location by channelling-enhanced microanalysis (ALCHEMI).

Phase equilibria and solid state transformations in Nb-rich Nb–Ti–Al intermetallic alloys

The solidification pathways and phase equilibria at 1100, 900 and 700°C in seven Nb-rich alloys of the Nb–Ti–Al system were examined. The as-cast microstructures revealed that all alloys had undergone β solidification, extending the range of the β phase field further into the ternary system than previously reported. Changes in the atomic site occupancy preferences with alloy composition were examined for the B2 ordered β0 phase through atom location by channeling-enhanced microanalysis (ALCHEMI). A β to δ massive-like transformation observed in cast material, was reproduced in continuously cooled homogenized samples. The formation of a new phase, θ, based on the I41/amd body centered tetragonal structure was observed below 1100°C. In addition, a metastable hP18 structure, produced through an ω-type transformation of the β0 phase, was also observed.

Optimizing the ALCHEMI technique

Abstract Systematic row atom location by channelling-enhanced microanalysis (ALCHEMI) experiments have been optimized theoretically with regard to dynamical interaction (beam voltage and reflecting planes), foil thickness and orientation. The optimum level of dynamical interaction is w s = 21/2 + Ξg/2Ξ2 g, the optimum orientation is the symmetry orientation and the optimum foil thickness lies between Ξg/4 and Ξg/3 when dynamical interaction is optimized. The corresponding maximum level of channelling is about 0.3–0.5, depending on absorption. Experimental measurements show good agreement with theory. It is shown experimentally that beam convergence may safely be increased up to the Bragg angle. As the level of dynamical interaction approaches that corresponding to the critical voltage, channelling (and the ease of ALCHEMI experiments) decreases to zero.

Locating dysprosium dopant atoms in the Bi2Sr2CaCu2Oy unit cell: Direct-space chemical crystallography

The position of Dy in the unit cell of Bi2Sr2CaCu2Oy has been determined by a real-space method. This is compared with the reciprocal-space atom location by channelling-enhanced microanalysis (ALCHEMI) method.

On the Effect of Antiphase Domain Boundaries on ALCHEMI

The effect of antiphase domain boundaries (APB) on ALCHEMI (atomic location by channelling-enhanced microanalysis) is discussed in the context of a two-beam dynamical approach. Appropriate experimental conditions for ALCHEMI in TEM foils with APBs are suggested.

On the Effect of Antiphase Domain Boundaries on ALCHEMI

The effect of antiphase domain boundaries (APB) on ALCHEMI (atomic location by channelling-enhanced microanalysis) is discussed in the context of a two-beam dynamical approach. Appropriate experimental conditions for ALCHEMI in TEM foils with APBs are suggested.

Alchemi Analysis of Cation Distributions in Spinel-Structured Compounds Using Oxygen as a Marker Element

The establishment of ALCHEMI (atom location by channeling-enhanced microanalysis) as a viable microanalytical technique owes much to its early success in determining cation distributions in crystals having the structure of the mineral spinel, MgAl2O4. The first application of ALCHEMI was to a synthetic chromite spinel, and geological applications to natural mineral spinels containing multiple 3d transition elements provided many of the early applications of the technique. Spinel has three distinct regular lattice sites, two cation sites and one anion site, occupying the 8a, 16d and 32e Wyckoff positions of the Fd3m space group. In projection along (800), tetrahedrally coordinated (8a) cations occupy one set of alternate planes, whereas octahedrally coordinated (16d) cations and, to a good approximation, the anions occupy the other set. In principle, therefore, ALCHEMI performed at an 800 planar channeling condition could be used to analyze site distributions in spinel, using oxygen as a marker for the planes containing the octahedral cations, as was early noted.

Site-distributions of Fe alloying additions to B2-ordered NiAl

The site-distributions of Fe alloying additions to B2-ordered NiAl alloys have been measured by ALCHEMI (atom location by channeling-enhanced microanalysis). Twelve alloy compositions were examined, of stoichiometries Ni50−xAl50Fex (Ni-deficient), Ni50Al50−xFex (Al-deficient), and Ni50−x/2Al50−x/2Fex (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.

Phase equilibria of L10-type order in Cu-Au-Pd and Cu-Au-Ni ternary systems

Abstract The IKL-ALCHEMI (intersecting Kikuchi-line-atom-location by electron channeling-enhanced X-ray microanalysis) method of electron diffraction was employed to study the atomic configuration in L10-type ordered phases in Cu-Au-Pd and Cu-Au-Ni ternary alloys annealed at 573 K. In the L10 phase of Cu-Au-Pd, Au and Pd atoms tend to share the same sublattice, while Cu atoms prefer the other sublattice. If the sum of Au and Pd contents exceeds 50 at.%, most Pd atoms still remain on the favourite sites, while excess Au atoms are driven to the opposite sublattice. In the ordered Cu-Au-Ni alloys, Ni atoms weakly prefer the Cu sublattice rather than the other for Au, but the distribution of Ni atoms depends on the alloy composition. Most Ni atoms reside on the Cu sublattice when the Au content is 50 at.%, while about two thirds of Ni atoms are found on the Au sublattice in the alloy with 50 at.% Cu. The cluster variation method (CVM) in the tetrahedral approximation explains well not only the experimental re...