Combination Of Electron Diffraction Research Articles

Cation Ordering in Layered O3 Li[NixLi1/3-2x/3Mn2/3-x/3]O2 (0 ≤ x ≤ 1/2) Compounds

Three layered compounds, Li[Ni1/2Mn1/2]O2 (x = 1/2), Li[Li1/9Ni1/3Mn5/9]O2 (x = 1/3), and Li2MnO3 (x = 0) of the Li[NixLi1/3-2x/3Mn2/3-x/3]O2 series were studied by electron diffraction in combination with synchrotron X-ray powder diffraction analyses. In-plane √3ahex. × √3ahex. ordering was found in all three samples, and a likely model for the occupancies of Li+, Ni2+, and Mn4+ on the sublattices of the transition metal layer was proposed. Perpendicular to the chex. axis, the ordered transition metal layers in the layered Li[Ni1/2Mn1/2]O2 and Li[Li1/9Ni1/3Mn5/9]O2 crystals could be arranged mostly in the P3112 stacking with some C2/m stacking (such as abab...) abnormally. It is believed that the nickel ions in the lithium layer of these two samples play an important role in the stacking of transition metal layers. Moreover, electron diffraction analyses suggested that individual Li2MnO3 crystals that were obtained at 850 °C had at least two different C2/m stacking sequence variants (abab..., caca..., an...

Photoelectron diffraction studies of Cu on Pd(111) random surface alloys

The study of surface alloys is motivated by their use in many applications of different segments of industry, such as in the search for new catalysts and sensors, in surface protection against corrosion, in lowering friction, and in testing electronic devices. An important aspect of surface alloys studies is that of the precise quantification of segregation and diffusion processes as well as the determination of surface structure. In this paper we report a combined low-energy electron diffraction and photoelectron diffraction (PED) (using synchrotron radiation) study of surface alloy formation when Cu ultrathin films are evaporated onto Pd(111) single-crystal surfaces. We present results for two different coverages (1 and 3 ML) and three annealing temperatures (300, 600, and $800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$). For these preparation conditions, a random alloy phase with different concentrations seems to form in the first few layers. Through the analysis of PED data performed using a multiple scattering formalism and the average $T$-matrix approximation it was possible to determine the atomic structure and the atomic concentration of the first three layers.

Structural phase transitions and sodium ordering in Na 0.5CoO 2: a combined electron diffraction and Raman spectroscopy study

The nonstoichiometric Na x CoO 2 system exhibits extraordinary physical properties that correlate with temperature and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na 0.7CoO 2, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na 0.33CoO 2·1.3H 2O is a newly discovered superconductor with T c∼4 K, and Na 0.5CoO 2 exhibits an unexpected charge ordering transition at around T co∼55 K. Recent studies suggest that the transport and magnetic properties in the Na x CoO 2 system strongly depend on the charge carrier density and local structural properties. Here we report a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na 0.5CoO 2 single crystals. A series of structural phase transitions in the temperature range from 80 to 1000 K are directly identified and the observed superstructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO 2 layers are a key factor for the presence of complex structural properties in Na x CoO 2 materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems.

Growth of SrTiO 3 films on Si(0 0 1)–Sr(2 × 1) surfaces

The growth and characterization of a SrTiO 3 film by molecular beam deposition process have been studied. After Sr deposition on the chemically formed SiO 2/Si surface, a stable and well-ordered Si(0 0 1)–Sr(2 × 1) surface was formed. The SrTiO 3 films were grown on the Si(0 0 1)–Sr(2 × 1) surface at 80 °C in a molecular oxygen partial pressure of approximately 3 × 10 −7 Torr and subsequently in situ post-annealed at various high temperatures without oxygen supply in ultra-high vacuum (<2.5 × 10 −9 Torr). The combined reflection high-energy electron diffraction, X-ray diffraction and atomic force microscopy analyses suggest that the SrTiO 3 film grown at 80 °C is amorphous nature. The lowest post-annealing temperature from amorphous-to-crystal transformation of the SrTiO 3 film is approximately 485 °C. The quality of the crystalline SrTiO 3 films is further improved by post-annealing at temperatures between 600 and 700 °C.

A combined temperature-dependent electron and single-crystal X-ray diffraction study of the fresnoite compound Rb2V4+V25+O8

High-purity Rb2V3O8 has been grown and temperature-dependent electron and single-crystal X-ray diffraction used to carefully investigate its fresnoite-type reciprocal lattice. In contrast to other recently investigated representatives of the fresnoite family of compounds, Rb2V3O8 is not incommensurately modulated with an incommensurate basal plane primary modulation wave vector given by q∼0.3 〈110〉*. A careful low-temperature electron diffraction study has, however, revealed the existence of weak incommensurate satellite reflections characterized by the primitive primary modulation wave vector q1∼0.16c*. The reciprocal space positioning of these incommensurate satellite reflections, the overall (3+1)-d superspace group symmetry, as well as the shapes of the refined displacement ellipsoids determined from single-crystal XRD refinement, are all consistent with their arising from a distinct type of condensed rigid unit modes (RUMs) of distortion of the Rb2V3O8 parent structure.

Electron density and implication for bonding in Cu

We report highly accurate measurements for the low-order Fourier components of the crystal potential in copper. These were obtained by transmission electron diffraction using a small probe and multiple scattering analysis. They were used to refine the $\text{Cu}\phantom{\rule{0.3em}{0ex}}3d$ radial wave function. An accurate charge-deformation map and a $3d$ orbital radial wave function were obtained by using a multipole refinement of the structure factors obtained from combined quantitative electron diffraction and $\ensuremath{\gamma}$-ray diffraction measurements. The results show a large change in the $3d$ orbital radial function from $d$-band formation and $d\ensuremath{-}s$ band crossing ($d\ensuremath{-}s$ hybridization). Band theory calculations are in excellent agreement with the measurements and show that the charge deformation in Ag is very similar to that in Cu. Our findings are in general agreement with the monovalent description of these metals.

Reversible phase changes of Ni-rich Al?Co?Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al–Co–Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al 70 Co 12 Ni 18 alloy, which is stable at 900 °C, transforms into the Type-I phase by annealing at 650 °C and that the Ni-rich basic phase formed in the Al 71.5 Co 9 Ni 19.5 alloy, which is stable at 900 °C, transforms into the S1-state phase by annealing at 650 °C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 °C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy.

Structure and Microstructure of Epitaxial Sr4Fe6O13±δ Films on SrTiO3

The crystal structure and the microstructure of epitaxial Sr4Fe6O13±δ thin films grown on a single-crystal SrTiO3 substrate by PLD have been investigated. A combination of electron diffraction and high-resolution microscopy allows us to refine the structure and to identify an incommensurate modulation in the Sr4Fe6O13±δ films. The incommensurate structure (q = αam* ≈ 0.39am*, superspace group Xmmm(α00)0s0) can be interpreted as an oxygen-deficient modification in the Fe2O2.5 double layers. Moreover, it is shown that the experimentally determined α component of the modulation can be used consistently to estimate the local oxygen content in the Sr4Fe6O13±δ films. The compound composition can therefore be described as Sr4Fe6O12+2α and the value α = 0.39 corresponds to a Sr4Fe6O12.78 composition. The misfit stress along the Sr4Fe6O13±δ/SrTiO3 interface is accommodated via both elastic deformation and inelastic mechanisms (misfit dislocations and 90° rotation twins). The present results also suggest the existe...

Kinetics of the heteroepitaxial growth of Ge layer at low temperature on Si(001) in UHV-CVD

The Ge growth at 330 °C by ultrahigh vacuum chemical vapour deposition is investigated in real time by reflection high energy electron diffraction (RHEED) in combination with atomic force microscopy and Rutherford back scattering spectrometry (RBS). The Stranski-Krastanov-related 2D to 3D transition is avoided at low temperature and the major part of the relaxation process occurs during the deposition of the first two monolayers. The very low growth rate observed during this first step is related to the deposition of Ge on Si. Beyond 2 deposited MLs, the growth rate increases drastically due to a complete coverage of Si by Ge. Finally, the deposition of Ge at 330 °C results in an in-plane lattice parameter approaching 90% of that of Ge bulk and a flat surface with rms roughness of 0.6 nm for a film thickness lower than 30 nm. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reversible phase changes of Ni-rich Al–Co–Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al–Co–Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al 70Co 12Ni 18 alloy, which is stable at 900 °C, transforms into the Type-I phase by annealing at 650 °C and that the Ni-rich basic phase formed in the Al 71.5Co 9Ni 19.5 alloy, which is stable at 900 °C, transforms into the S1-state phase by annealing at 650 °C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 °C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy.

Laser-driven formation of a high-pressure phase in amorphous silica.

Because of its simple composition, vast availability in pure form and ease of processing, vitreous silica is often used as a model to study the physics of amorphous solids. Research in amorphous silica is also motivated by its ubiquity in modern technology, a prominent example being as bulk material in transmissive and diffractive optics for high-power laser applications such as inertial confinement fusion (ICF). In these applications, stability under high-fluence laser irradiation is a key requirement, with optical breakdown occurring when the fluence of the beam is higher than the laser-induced damage threshold (LIDT) of the material. The optical strength of polished fused silica transmissive optics is limited by their surface LIDT. Surface optical breakdown is accompanied by densification, formation of point defects, cratering, material ejection, melting and cracking. Through a combination of electron diffraction and infrared reflectance measurements we show here that synthetic vitreous silica transforms partially into a defective form of the high-pressure stishovite phase under high-intensity (GW cm(-2)) laser irradiation. This phase transformation offers one suitable mechanism by which laser-induced damage grows catastrophically once initiated, thereby dramatically shortening the service lifetime of optics used for high-power photonics.

Strain determination in silicon microstructures by combined convergent beam electron diffraction, process simulation, and micro-Raman spectroscopy

Test structures consisting of shallow trench isolation (STI) structures are fabricated using advanced silicon (Si) technology. Different process parameters and geometrical features are implemented to investigate the residual mechanical stress in the structures. A technology computer aided design homemade tool, IMPACT, is upgraded and optimized to yield strain fields in deep submicron complementary metal–oxide–semiconductor devices. Residual strain in the silicon substrate is measured with micro-Raman spectroscopy (μ-RS) and/or convergent beam electron diffraction (CBED) for large (25 μm) and medium size (2 μm), while only CBED is used for deep submicron STI (0.22 μm). We propose a methodology combining CBED and technology computer aided design (TCAD) with μ-RS to assess the accuracy of the CBED measurements and TCAD calculations on the widest structures. The method is extended to measure (by CBED) and calculate (by TCAD) the strain tensor in the smallest structures, out of the reach of the μ-RS technique. The capability of determining, by both measurement and calculation, the strain field distribution in the active regions of deep submicron devices is demonstrated. In particular, it is found that for these structures an elastoplastic model for Si relaxation must be assumed.

A combined diffraction (XRD, electron and neutron) and electrical study of Na 3MoO 3F 3

Na 3MoO 3F 3, a member of the A 2 BM VIO 3F 3 family of elpasolite-related oxyfluorides, has been prepared by the reaction of NaF with MoO 3 at 650°C. It is shown by a combined X-ray, electron and neutron diffraction study, that the true symmetry of Na 3MoO 3F 3 is not monoclinic (pseudo-orthorhombic) as previously reported but instead triclinic (metrically rhombohedral) P1. The superstructure unit cell is given by a=− a R+ b R, b= c R, c=( a R+ b R+ c R), when expressed with respect to the underlying rhombohedral parent structure. Neutron diffraction refinement of the rhombohedral sub-structure shows O and F to be fully ordered with alternate (111) R planes being occupied by O and then F. The nature of the distortion away from the high temperature Fm 3 ̄ m parent structure is consistent with a combination of φφφ type rotations of the octahedral framework together with ppp type displacements of the octahedral cations, which is not observed in any of the closely related A 2 BM VIO 3F 3 family members. Bond valence arguments are used to give a plausible explanation for this difference in behavior.

Atomic ordering in the doped rare earth cobaltates Ln0.33Sr 0.67CoO 3− δ ( Ln=Y 3+, Ho 3+ and Dy 3+)

The perovskite-based rare earth cobaltates ( Ln 0.33Sr 0.67CoO 3− δ ) ( Ln=Y 3+, Ho 3+ and Dy 3+) have been synthesized at 1100°C under 1 atm oxygen. A thermogravimetric study has determined the overall oxygen content in each case while a combined electron diffraction (ED) and synchrotron X-ray diffraction study has revealed the presence of a complex, previously unreported, perovskite-related superstructure phase. ED gave a resultant C1 c1 but most probably Cmcm ( a =2a p −2c p , b =4b p , c =2a p +2c p ) perovskite-related superstructure, describable as a modulated I4/ mmm intermediate parent structure. Synchrotron X-ray data has been used to refine the intermediate parent structures of all three compounds. Coupled Ln/Sr and O/vacancy ordering and associated structural relaxation is shown to be responsible for the observed superstructure.

The molecular structure of the anti conformation of vinylcyclopropane as determined by combined electron diffraction and microwave analysis

The electron diffraction data for vinylcyclopropane collected at 273 K and the rotational constants for the parent and four 13C isotopic species of the anti form were analyzed. A mixture of anti (76(10)%) and gauche forms was observed in the vapor phase. The geometrical parameters ( r g/∠ α obtained from least squares analysis are as follows: r( C– H) av =1.098(3) A ̊ , r( C 1– C 2)=1.521(4) A ̊ , r( C 1– C 2)−r( C 2– C 3)=0.011(6) A ̊ , r( C 1– C 4)=1.481(7) A ̊ , r( C 4= C 5)=1.340(2) A ̊ , ∠C 3C 1C 4=118.2°(5), ∠C 1C 4=C 5=125.8°(6), ∠HC= 4C 5=116.8°(25), ∠(HCC) C3=118.0°(6), τH 6C 1C= 4C 5=0.0° (anti);124° (18) (gauche) and %anti=76(10).

Effect of diffraction crystallography on HREM

Abstract A simple image contrast theory in high-resolution electron microscopy (HREM) is introduced to demonstrate that below a certain critical crystal thickness the intensity of the Scherzer focus image is linear to the projected potential of an artificial crystal that is isomorphic to the examined one. It has become the theoretical base of electron crystallographic image processing techniques relying on the weak-phase-object approximation and kinematical diffraction. Two techniques of image processing are introduced. One of them aims at determining crystal structures by combining electron diffraction data and applying diffraction analysis methods. To reduce various kinds of electron diffraction intensity distortion the diffraction data are corrected by means of an empirical method set up by referring to the heavy atom method and Wilson statistic. The other one aims at revealing crystal defects at atomic resolution from the image taken with a medium-voltage field-emission high-resolution electron microscope. The dynamical effect is corrected by forcing the integral amplitudes of reflections in the diffractogram of image equal to the amplitudes of corresponding structure factors for the perfect crystal. The principle of the two techniques is briefly introduced, and examples of applications to crystal structure and defect determination are given.

Domino phase-retrieval algorithm for structure determination using electron diffraction and high-resolution transmission electron microscopy patterns.

Direct-method formalism to determine atomic structures using electron diffraction data is here aimed at a general solution of the phase-retrieval problem, consequently combining electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) patterns in a 'domino' fashion. While there are similarities to conventional (kinematical) direct methods, there remain major differences; in particular, owing to the dynamical effects in the data, the ED structure factors prove to be complex and then the positivity of the reconstructed electron density is no longer a valid constraint for 'dynamical' direct methods. Besides, owing to the dynamical effects, heavy atoms no longer dominantly contribute to the HRTEM images. Thus, the 'dynamical' direct-methods concept is based on the phase-retrieval algorithm utilizing both the dynamical ED and the HRTEM data. The fusion of the traditional direct-method technique, which is described here, allows realization of a full-phase restoration of complex structure factors. A numerical example, using the dynamical ED and HRTEM data for (Ga,In)(2)SnO(5) ceramic, shows that the method is capable of yielding a unique phase-retrieval solution. The clear sense is that the domino transform algorithm proposed works well and represents a valuable method for phasing diffraction patterns in electron structural crystallography using an experiment that is readily performed when the ED and HRTEM data are collected.

Structural Study of an Unusual Cubic Pyrochlore Bi 1.5Zn 0.92Nb 1.5O 6.92

Single-phase pyrochlore-type specimens of Bi 1.5Zn 0.92Nb 1.5O 6.92 were studied using combined electron, X-ray and neutron powder diffraction techniques. Rietveld refinements using neutron powder diffraction data confirmed an average pyrochlore structure A 2 B 2O 6O′ ( Fd&3macr; m, a=10.5616(1) Å) with both Bi and Zn mixed on the A-sites. However, refinements revealed significant local deviations from the ideal pyrochlore arrangement which were caused by apparent displacive disorder on both the A and O′ sites. The best fit was obtained with a disordered model in which the A-cations were randomly displaced by ∼0.39 Å from the ideal eight-fold coordinated positions. The displacements occur along the six 〈112〉 directions perpendicular to the O′– A–O′ links. In addition, the O′ ions were randomly displaced by ∼0.46 Å along all 12 〈110〉 directions. Crystal–chemical considerations suggest the existence of short-range correlations between the O′ displacements and both the occupancy of the A-sites (i.e., Bi or Zn) and the directions of the A-cation displacements. The combined A-cation and O′ displacements change the coordination sphere of the A-cations from 8 to (5+3); the resulting coordination environment of the A-cations bears similarities to that of the (5+1)coordinated Zn in zirconolite-like Bi 2Zn 2/3Nb 4/3O 7. The observed displacive disorder in the A 2O′ network of the Bi 1.5Zn 0.92Nb 1.5O 6.92 structure involves atoms associated with the lowest-frequency vibrational bending mode, and is likely responsible for both the high dielectric constant and the dielectric relaxation reported for this compound.

X-Ray and Electron Microscopy Studies of Rare-Earth Tungsten Bronzes

Rare-earth tungsten bronzes, RE x WO 3, of perovskite tungsten-bronze (PTB) type, formed by conventional solid-state synthesis, have been studied by X-ray powder diffraction, electron diffraction in combination with microanalysis, and high-resolution transmission electron microscopy (HRTEM). The X-ray patterns indicated cubic PTB type structure with a≈3.83 Å (subcell), while the electron microscopy study indicated a lowering of the subcell symmetry and a complex superstructure. The upper phase composition limit, x≤0.25, for the RE x WO 3 bronzes with PTB-related structures was established from the microanalysis study. Ordered, disordered and microtwinned superstructures were revealed by electron diffraction patterns and HRTEM images taken along 〈110〉 p. The superstructure is due to filling of the interstices in the WO 3 structure with the rare-earth ions. A hypothetical model of the superstructure based on the contrast features in the HRTEM images has been deduced. The relationships between the RE x WO 3 bronzes formed by solid-state synthesis under high- and ambient-pressure conditions are presented.

X-Ray and Electron Microscopy Studies of Rare-Earth Tungsten Bronzes

Rare-earth tungsten bronzes, RExWO3, of perovskite tungsten-bronze (PTB) type, formed by conventional solid-state synthesis, have been studied by X-ray powder diffraction, electron diffraction in combination with microanalysis, and high-resolution transmission electron microscopy (HRTEM). The X-ray patterns indicated cubic PTB type structure with a≈3.83 Å (subcell), while the electron microscopy study indicated a lowering of the subcell symmetry and a complex superstructure. The upper phase composition limit, x≤0.25, for the RExWO3 bronzes with PTB-related structures was established from the microanalysis study. Ordered, disordered and microtwinned superstructures were revealed by electron diffraction patterns and HRTEM images taken along 〈110〉p. The superstructure is due to filling of the interstices in the WO3 structure with the rare-earth ions. A hypothetical model of the superstructure based on the contrast features in the HRTEM images has been deduced. The relationships between the RExWO3 bronzes formed by solid-state synthesis under high- and ambient-pressure conditions are presented.