Electron Microdiffraction Research Articles

An efficient approach to characterize pseudo-merohedral twins by precession electron diffraction: Application to the LaGaO 3 perovskite

Pseudo-merohedral twins are frequently observed in crystals displaying pseudo-symmetry. In these crystals, many [ u v w] zone axis electron diffraction patterns are very close and can only be distinguished from intensity considerations. On conventional diffraction patterns (selected-area electron diffraction or microdiffraction), a strong dynamical behaviour averages the diffracted intensities so that only the positions of the reflections on a pattern can be considered. On precession electron diffraction patterns, the diffracted beams display an integrated intensity and a “few-beam” or “systematic row” behaviour prevails which strongly reduces the dynamical interactions. Therefore the diffracted intensity can be taken into account. A procedure based on observation of the weak extra-reflections connected with the pseudo-symmetry is given to identify without ambiguity any zone axis. It is successfully applied to the identification and characterization of {1 2 1} reflection twins present in the LaGaO 3 perovskite.

10 MeV Au ion irradiation effects in an MgO–HfO 2 ceramic–ceramic (CERCER) composite

Room temperature ion irradiation damage studies were performed on a ceramic composite intended to emulate a dispersion nuclear fuel. The composite is composed of 90-mole% MgO and 10-mole% HfO 2. The as-synthesized composite was found to consist of Mg 2Hf 5O 12 (and some residual HfO 2) particles embedded in an MgO matrix. X-ray diffraction revealed that nearly all of the initial HfO 2 reacted with some MgO to form Mg 2Hf 5O 12 . Ion irradiations were performed using 10 MeV Au 3+ ions at room temperature over a fluence range of 5 × 10 16–5 × 10 20 Au/m 2. Irradiated samples were characterized using both grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM), the latter using both selected-area electron diffraction (SAED) and micro-diffraction (μD) on samples prepared in cross-sectional geometry. Both GIXRD and TEM electron diffraction measurements on a specimen irradiated to a fluence of 5 × 10 20 Au/cm 2, revealed that the initial rhombohedral Mg 2Hf 5O 12 phase was transformed into a cubic-Mg 2Hf 5O 12 phase. Finally, it is important to note that at the highest ion fluence used in this investigation (5 × 10 20 Au/m 2), both the MgO matrix and the Mg 2Hf 5O 12 second phase remained crystalline.

Methods of electron microdiffraction and X-ray analysis in structure study of nanodisperse partially stabilized ZrO2 powders

Analytical electron microscopy (AEM) has been used to study both structure and morphology of partially yttria-stabilized zirconia dioxide nanopowders (YSZ) obtained by wet-chemical methods (glycine and azeotropic distillation) and ceramics produced from them. Both morphological and structural inhomogeneity of nanopowders obtained by glycine (glc) method has been estimated. Besides the tetragonal ZrO2 phase (results of X-ray analyses) the cubic phase of ZrO2 with different degree of crystallinity has been estimated by Electron Microdiffraction (EMD) methods. In powders obtained by azeotropic distillation (dest) method besides the amorphous phase (identified in X-ray investigations) the high disperse cubic zirconia phase has been identified using high local EMD method. It has been detected the yttrium influence on the degree of crystallinity in nanopowders obtained by azeotropic distillation method without yttria (dest-0YSZ) and with 5 wt % Y2O3 (dest-5YSZ). It has been determined the difference in ceramic morphology produced from these powders. Ceramics mode of nanopowders containing yttria (glc-5YSZ and dest-5YSZ) have a homogeneous surface which consists of different size globules (0.1–0.6 μm) and contains some little pores (∼370 nm). Ceramics mode of nanopowders without yttria have inhomogeneous surface with numerous cracks. Separate parts of the latter ceramics consist of globules, their sizes are of 0.2–0.5 μm.

Phases formed during rapid quenching of liquid carbon

Pulsed laser action upon a sample of highly oriented pyrolytic graphite (HOPG) in a gasostat filled with helium at a pressure above that corresponding to the triple point of carbon, followed by rapid quenching of the liquid phase at a rate of about 106 K/s leads to the formation of a crater with a periodic spatial structure at the surface. The composition and structure of nongraphite carbon phases in the near-surface region of the crater have been studied using the Raman scattering spectroscopy, electron microdiffraction, and energy-dispersive X-ray analysis. It is established that rapidly quenched carbon possesses predominantly a hybrid structure of glassy carbon formed as a result of the high-temperature treatment, with inclusions of crystalline carbyne, chaoite, and a hybrid cubic phase of ultradense carbon (C8). The hybrid phases of glassy carbon and C8 had not been reported until now as possible products of solidification of liquid carbon.

Pearlite in hypoeutectoid iron–nitrogen binary alloys

In this study, hypoeutectoid Fe–N binary specimens have been prepared by gas nitriding pure iron in austenite domain at 840 °C. The slow cooling of these specimens led to the α-ferrite + γ′-Fe4N pearlitic microstructure which is similar to the pearlite in Fe–C binary system. This pearlitic microstructure has been characterized by electron microscopy. The crystal structure of the γ′-Fe4N nitride has been identified by electron microdiffraction and the Nishiyama–Wassermann (N–W) and near Kurdjumov–Sachs (K–S) orientation relationships have been found between the α-ferrite and the γ′-Fe4N.

Transmission electron microscopy of Zr–Zn precipitate rods in magnesium alloys containing Zr and Zn

Rod-shaped Zr–Zn particles are commonly observed within individual α-Mg grains in magnesium alloys containing both Zr and Zn elements. In the present study, the identity, morphology, and orientation of such solid-state precipitate particles formed in a Mg–1Ca–1Zn–1Zr (wt.%) alloy have been characterized using analytical transmission electron microscopy and electron microdiffraction. It is found that these particles have a tetragonal crystal structure similar to that of Zn2Zr3 (P42/mnm, a = 0.768 nm, c = 0.699 nm). Three distinguishably different orientation relationships are observed between the Zn2Zr3 and matrix phases. But the long axes of the rods are invariably parallel to the [001] direction of the Zn2Zr3 phase.

Formation of magnetic nanocrystals in germanate glasses doped with Fe and Dy

Magnetic nanocomposites were synthesized from germanate glasses doped with iron and dysprosium oxides at low concentrations. The formation of nanoparticles (∼5–20 nm in size) in glasses subjected to heat treatment under different conditions was observed using high-resolution electron microscopy. Nanoparticles had a well-resolved crystal structure. X-ray fluorescence microprobe analysis showed that iron and dysprosium are concentrated in particles. The examination of the field dependences of the magneto-optical effect revealed magnetic ordering in particles. A comparison of the interplanar distances obtained using electron microdiffraction with those of the known iron oxide compounds did not lead to unambiguous results. Almost all specific features in the magnetic circular dichroism spectra of the glasses corresponded to the features previously observed in the spectra of garnet ferrites. On this basis, it was assumed that the particles formed in the germanate glasses have a distorted garnet structure.

Light ion irradiation-induced phase transformation in the monoclinic polymorph of zirconia

Ion irradiation damage experiments were performed at ∼80 K on polycrystalline samples of monoclinic, slightly sub-stoichiometric zirconia (ZrO 1.98). Following irradiation with 150 keV Ne + ions, the monoclinic phase was gradually replaced by a new phase. Transmission electron microscopy (TEM) observations in cross-sectional geometry and electron microdiffraction (μD) measurements revealed that the irradiated layer in a sample irradiated to a fluence of 5 × 10 20 Ne/m 2 is partially transformed to a higher symmetry phase of high crystallinity. This phase transformation is accompanied by reduction of the initial micron-sized, highly-twinned grain distribution, to a nano-phased grain structure. Grazing incidence X-ray diffraction (GIXRD) measurements revealed that the radiation-induced phase is a tetragonal polymorph of zirconia. This was verified by the existence of strong (1 0 1) diffraction maxima and weak (1 0 2) reflections (body-centered cell). Raman spectroscopy (RS) measurements were also performed in an attempt to corroborate GIXRD results obtained from the irradiated material. RS measurements in the confocal geometry agreed with GIXRD measurements, although RS was not as definitive as GIXRD. In addition to RS showing the existence of a band corresponding to a tetragonal structure at 262 cm −1, a new mystery band appeared at 702 cm −1 that increased in intensity as a function of irradiation fluence.

M 2N nitride phases of 9% chromium steels for nuclear applications

M 2N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600–780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M 2N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 Å/ c = 4.45 Å and a = 7.76 Å/ c = 4.438 Å, were determined in the steels. Four types of Cr-rich M 2N phases with different lattice parameters probably existed in the steels. The M 2N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.

Carbonaceous substance of the Sukhoi Log gold deposit ( East Siberia)

Insoluble carbonaceous substance (ICS) at the Sukhoi Log gold deposit has been studied following the scheme: source shales — concentrates — residual substance of rocks. Examination by electron microscopy, transmission electron microscopy with electron microdiffraction, thermography, and mass spectrometry revealed several morphogenetic ICS varieties: dot-drop-like, honeycomb, single graphite crystals, and spherical graphite crystals. Study of source shales and concentrates has shown their similar carbon isotope compositions (δ 13C av = 18.03–17.54‰). Residual carbonaceous substance is characterized by a heavy carbon isotope composition (δ 13C av = 10‰). Its enrichment with heavy carbon isotope, as compared with the source rocks, calls for special geochemical studies. The ICS heterogeneity in the Sukhoi Log shales points to variations in the physicochemical settings of ore formation at the deposit.

Structure−Property Relationships of the 10H Hexagonal-Type Perovskite BaMn0.4Fe0.6O2.73

The crystal structure of BaMn0.4Fe0.6O2.73, a recently identified oxygen-deficient 10H-type hexagonal perovskite in the system BaMn1-xFexO3-y, has been established by a combination of electron microdiffraction and neutron diffraction. The structure (space group P63/mmc, a = 5.74435(5) Å, and c = 24.0331(3) Å) can be described by a stacking sequence (hch‘ch)2 along the c-axis with the anion deficiency located exclusively in the h‘-BaO3-type layers. The anion distribution in the h‘-BaO3 layers differs significantly from that observed for 10H BaFeO2.8 and results in a 70:30 random distribution of corner-sharing tetrahedral Fe2O7 dimers and face-sharing octahedral (Mn,Fe)2O9 dimers as opposed to the exclusive Fe2O7 dimers in 10H BaFeO2.8. The difference is attributed to the preference of Mn for octahedral coordination. The compound is a “leaky” insulator at room temperature with a permittivity of ∼20. The conduction mechanism has low activation energy, ∼0.3 eV, and is consistent with polaronic hopping associated with the Fe and/or Mn ions.

Precipitated iron Fischer-Tropsch catalyst: Effect of carbidization on the morphology of iron oxyhydroxide nanoneedles

The effect of carbidization on the morphology of iron oxyhydroxide particles has been studied. Homogeneous oxidation of iron(II) sulfate results in predominantly γ-FeOOH particles which have a rod-shaped crystalline structure. Carbidization of the prepared iron oxyhydroxide particles at 270 °C and atmospheric pressure without mechanical stirring was found to produce iron carbides with spherical layered structure residing in close proximity. HRTEM, EDS and electron microdiffraction analysis revealed that carbidization alters the initial nanoneedle-type morphology and generates ultrafine carbide particles with irregular spherical shape. When carbidization was performed under severe mechanical stirring, formation of spherical carbide particles completely separated from each other was obsrved. Initial activity of iron oxyhydroxide for Fischer-Tropsch synthesis was found to be high (a CO conversion of about 84%) which declined steadily with synthesis time (26% CO conversion after 340 h of synthesis). The effect of carbidation on the morphology of iron hydroxide oxide (γ-FeOOH) particles with a three dimensional crystalline nanoneedle structure has studied. Carbidation of γ-FeOOH with CO at atmospheric pressure produced iron carbides with spherical layered structure. HRTEM and EDS analysis revealed that carbidation changes the initial nanoneedles morphology and generates ultrafine carbide particles with irregular spherical shape.

TEM characterization of organic nanocrystals grown in sol–gel thin films

The tetracene molecule (2,3-benzanthracene, C8H12) was used to synthesize nanocrystals grown in sol–gel thin films, ranging from 10 to 100 nm of diameter. This confined nucleation and growth was compared to microcrystallizations of the same molecule in free solution. Transmission electron microscopy (TEM) was used to characterize these two kinds of tetracene crystals. The observation was performed under low-dose illumination to avoid amorphization of the samples during electron irradiation. Spatial confinement and size distribution of micro- and nanocrystals were compared. Using electron microdiffraction and diffraction patterns simulations, we showed that free microcrystals and nanocrystals confined in gel glasses exhibit the same triclinic $$ P\overline 1 $$ structure. In addition, time-resolved spectroscopy was used to record fluorescence decays, showing a monoexponential fluorescence decay for nanocrystals while microcrystals exhibit a multiexponential decay. The simple signature of nanocrystals luminescence is promising for the future development of chemical or biological sensors.

LACDIF, a new electron diffraction technique obtained with the LACBED configuration and a Cs corrector: Comparison with electron precession

By combining the large-angle convergent-beam electron diffraction (LACBED) configuration together with a microscope equipped with a C s corrector it is possible to obtain good quality spot patterns in image mode and not in diffraction mode as it is usually the case. These patterns have two main advantages with respect to the conventional selected-area electron diffraction (SAED) or microdiffraction patterns. They display a much larger number of reflections and the diffracted intensity is the integrated intensity. These patterns have strong similarities with the electron precession patterns and they can be used for various applications like the identification of the possible space groups of a crystal from observations of the Laue zones or the ab-initio structure identifications. Since this is a defocused method, another important application concerns the analysis of electron beam-sensitive materials. Successful applications to polymers are given in the present paper to prove the validity of this method with regards to these materials.

A modular model of the crystal structure of the pyrochlore-murataite polysomatic series

This paper reports on the results of a structural investigation of an important group of synthetic compounds with derivative structures of the mineral murataite, a potential matrix for immobilization of radioactive wastes. A model describing the structure of the entire group of compounds is proposed on the basis of analyzing the results obtained using chemical (electron microprobe) analysis methods, high-resolution transmission electron microscopy (high-resolution electron images and microdiffraction patterns), and X-ray diffractometry of a large array of artificial murataite ceramics. In essence, the proposed structural model is as follows: in the structure, layers of two (pyrochlore and murataite) packing types alternate along the [111] direction of the cubic supercell, so that the resulting structures can undergo substitutional and displacive modulations. Experiments on irradiation of different murataite modifications with heavy ions demonstrate that these modifications have close values of the radiation resistance, which is higher than that of the pyrochlore coexisting with them.

Characterization of strengthening precipitate phases in a Mg–Zn alloy

Strengthening precipitate phases in a Mg–8 wt.% Zn alloy, aged isothermally at 200 °C, have been characterized using transmission electron microscopy and electron microdiffraction. Precipitation at 200 °C involves formation of β 1 ′ and β 2 ′ transition phases. Contrary to previous studies, the β 1 ′ phase is found to have a monoclinic structure similar to that of Mg4Zn7. The β 2 ′ phase is confirmed to have a hexagonal structure similar to that of MgZn2.

Identification of Quasicrystalline Particles in an Isothermally Aged Al-10Mg-0.5Ag Alloy

The trace additions of Ag to binary Al-Mg alloys promote precipitation of a phase in the under-aged condition. The structure of the phase in an Al-10Mg-0.5Ag (mass%) alloy have been characterised using transmission electron microscopy and electron microdiffraction. An icosahedral quasicrystalline phase, which is identified by the electron microdiffraction patterns, has been found in the Al-10Mg-0.5Ag (mass%) alloy after solution treatment, water quenched and then aged during the time between 20 and 40 min. at 513K. The orientation relationship between the quasicrystalline phase and the a - Al matrix is as follows; i5 // <011>a and i3 // <111>a. A uniform distribution of finer-scale (typically 20 nm), faceted precipitate icosahedral quasicrystalline particles was observed within the core of the grains, but the precipitates became coarser (60-80 nm) approaching the grain boundary precipitate-free zone. The quasicrystalline particles contain elements of Al, Mg and Ag. The quasicrystalline precipitate particles, which are the metastable phase, appear to be the primary strengthening phase in the Al-10Mg-0.5Ag (mass%) alloy aged at 513K.

Hydrogen-implant-induced polarization loss and recovery in IrO2∕Pb(Zr,Ti)O3∕Pt capacitors

Hydrogen was implanted into IrO2 (200nm)∕Pb(Zr,Ti)O3∕Pt thin film capacitors at 26keV with a flux of 2×1015H+ions∕cm2 and also implanted at 20keV with a flux of 1×1015H+ions∕cm2 into IrO2 (50nm)∕Pb(Zr,Ti)O3∕Pt. The implanted samples showed a net loss in polarization of 20μC∕cm2 as a result of the hydrogen implant and the hysteresis loop was severely distorted. Transmission electron microscopy cross-sectional images and microdiffraction patterns showed no physical damage in the Pb(Zr,Ti)O3 after implanting. Secondary ion mass spectroscopy (SIMS) analysis revealed that a maximum concentration of H of 4×1020atom∕cm3 after implantation. Permittivity measurements also showed a decrease after implantation of 20% or more. After annealing the implanted samples in air at 350°C for 5min the remanent polarization loop and dielectric properties were restored. SRIM-2003 software simulation of the implanted hydrogen revealed that 1% oxygen vacancies are created by implanting using these implant conditions. In addition, hydrogen implanted in the Pb(Zr,Ti)O3 lattice can inhibit the switching of Ti∕Zr by binding with oxygen. The recovery at 350°C indicates that the polarization decrease created by hydrogen implantation is relatively minor and suggests that the degradation mechanism is due to implanted hydrogen in the Pb(Zr,Ti)O3 films.

Tetragonal Phase in Al‐Rich Region of U—Fe—Al System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Diffractive electron imaging of nanoparticles on a substrate

The observation of the detailed atomic arrangement within nanostructures has previously required the use of an electron microscope for imaging. The development of diffractive (lensless) imaging in X-ray science and electron microscopy using ab initio phase retrieval provides a promising tool for nanostructural characterization. We show that it is possible experimentally to reconstruct the atomic-resolution complex image (exit-face wavefunction) of a small particle lying on a thin carbon substrate from its electron microdiffraction pattern alone. We use a modified iterative charge-flipping algorithm and an estimate of the complex substrate image is subtracted at each iteration. The diffraction pattern is recorded using a parallel beam with a diameter of approximately 50 nm, illuminating a gold nanoparticle of approximately 13.6 nm diameter. Prior knowledge of the boundary of the object is not required. The method has the advantage that the reconstructed exit-face wavefunction is free of the aberrations of the objective lens normally used in the microscope, whereas resolution is limited only by thermal vibration and noise.