An electron microscopy investigation of tin-molybdenum oxides

Tin-molybdenum oxides prepared by the calcination of precipitates at high temperatures have been investigated by electron microscopy. The solids, which adopt the rutile-related tin(IV) oxide-type structure, are composed of crystalline particles containing planar defects, some of which have been characterized as twin boundaries. Although molybdenum has been found to be concentrated at these planar faults the examination of pure tin(IV) oxide prepared by similar methods shows that their formation is not dependent on the presence of molybdenum. It is suggested that molybdenum segregates to the twin boundaries during the calcination process to accommodate itself in more favorable coordination.     

X-ray microanalysis and high-resolution transmission electron microscopy of the reduced titanium-niobium oxides

Phase relationships in TiNb₂O₇ and Ti₂Nb₁₀O₂₉ reductions at 1400°C were investigated by means of X-ray microanalytical electron microscopy and high-resolution transmission electron microscopy (TEM). Compositions of phases present in equilibrium were obtained by applying thin-crystal approximation by which $\text{Nb}\text{Ti}$ ratios in different phases were determined; their oxygen content was inferred from structural considerations. In this manner, phase relationships in that portion of the TiO₂NbO₂NbO_2.5 equilibrium diagram with 2.417 ≥ x (in MeO_x) ≥ 2 were defined. Data obtained, in combination with high-resolution electron microscopy observations, confirmed that the reduction reaction, in part, is a heterogeneous process controlled by outward diffusion of both metal and oxygen atoms. Recombination of the diffused particles leads to the formation of separate crystals. The original block structure phase undergoes transformation in a quasihomogeneous manner either to an isomorphous phase in the binary NbO system or to a structurally related lower composition oxide. A new superstructure Me₂₅O₆₀(Ti_7.16Nb_42.84O₁₂₀) has been detected as an intermediate metastable phase, generated in the reduction of TiNb₂O₇ to stable Me₁₂O₂₉(Ti_1.53Nb_10.47O₂₉) and MeO₂(Ti_0.52Nb_0.48O₂) phases. Consideration of phase relationships among Me₂₅O₆₀, Me₁₂O₂₉, and MeO₂ suggests a chemical mechanism for the reaction concerned. The Me₂₅O₆₀ superstructure has a monoclinic symmetry with cell parameters a = 19.0 Å, b = 3.8 Å, c = 26.6 Å, α = 90°, β = 90°, γ = 78.5°, as determined from the structure image calculations.     

Investigations of the tin-antimony-oxygen system by high-resolution electron microscopy

A range of tin-antimony oxides, prepared by the calcination of precipitates, were examined by high-resolution electron microscopy. Products formed at 600°C contain small crystals of a rutile-type material and, depending on antimony concentration, varying amounts of disordered and/or amorphous phases. The observations are consistent with a resistivity to bulk phase equilibrium under conditions of low temperature and high antimony concentrations. Heating of the tin-antimony oxides to 1000°C for prolonged periods is accompanied by an increase in the crystallinity and particle size of the rutile-type material as a result of the thermally induced aggregation of tin(IV) oxide units. The observations are consistent with limited antimony incorporation in the bulk tin(IV) oxide lattice and a migration of antimony to surface sites. There was no evidence for any discrete, readily identifiable, antimony oxide phases, although a nonrutile-type material was observed at higher antimony concentration. The rutile-type phases often contained planar faults which were identified in some instances as twin boundaries; the possibility that these might provide a means of accommodating antimony within the tin oxide lattice is briefly considered. The relationship between our observations and the information available from other techniques is discussed.     

Three-dimensional imaging of YB56 by high-resolution electron microscopy

The three-dimensional potential map of YB56 was obtained by inverse Fourier transformation of three-dimensional phases and amplitudes in three high-resolution images taken along the [100], [110] and [111] directions of YB56 crystals; the size of the imaging region was 14 nm x 14 nm x approximately 4 nm, and the image directly showed the three-dimensional potential map of the crystal, a useful method for three-dimensional structure analysis in nanoscale regions.     

Nanometre resolution using high-resolution scanning electron microscopy corroborated by atomic force microscopy

The resolving power of high-resolution scanning electron microscopy was judged using topographical height data from atomic force microscopy in order to assess the technique as a tool for understanding nanoporous crystal growth.     

Quantitative high-resolution transmission electron microscopy of single atoms

Single atoms can be considered as the most basic objects for electron microscopy to test the microscope performance and basic concepts for modeling image contrast. In this work high-resolution transmission electron microscopy was applied to image single platinum, molybdenum, and titanium atoms in an aberration-corrected transmission electron microscope. The atoms are deposited on a self-assembled monolayer substrate that induces only negligible contrast. Single-atom contrast simulations were performed on the basis of Weickenmeier-Kohl and Doyle-Turner form factors. Experimental and simulated image intensities are in quantitative agreement on an absolute intensity scale, which is provided by the vacuum image intensity. This demonstrates that direct testing of basic properties such as form factors becomes feasible.     

High-resolution electron microscopy of crystallographic shear structures in tungsten oxides

Crystallographic shear (CS) structure in reduced crystals of W0₃ has been imaged at a resolution of 3–4A?by a high-resolution electron microscope.A?large distortion of the W0₆ octahedra sharing their edges at the CS planes has been directly recognized in electron micrographs.Crystallographic shear occurs preferentially in a particular crystallographic orientation. The preference may be explained by a different degree of distortion along the principal axes in the W0₆ octahedra of the pseudocubic structure of W03 crystal.A model for growth and ordering of the CS planes is discussed.     

High-resolution transmission electron microscopy—Investigation of vanadium-tungsten oxides prepared by chemical transport reactions

Crystalline monophasic samples of three mixed vanadium-tungsten oxides (V_0.8W_0.2)₃O₇ ? (V,W)₉O₂₁, (V_0.65W_0.35)₂O₅ ? (V,W)₁₆O₄₀, and V_0.64W_0.36O_2.60 ? V₁₆W₉O₆₅ were prepared by chemical transport reactions. The block structures of these compounds were investigated by high-resolution electron microscopy. They are built up by square blocks with [3 × 3 × ∞], [4 × 4 × ∞], or [5 × 5 × ∞] cornersharing MO-octahedra. Other block sizes were only observed as defects. The thermal behavior of the compounds was investigated.     

Observation of the verwey transition in Fe3O4 by high-resolution electron microscopy

High resolution electron microscopy (HREM) of Fe₃O₄ has been carried out at temperatures near the Verwey transition (∼120 K) with a point resolution of 3 Å. Lattice fringes of both the high- and the low-temperature phases have been observed at these temperatures. The crystal symmetry of the low-temperature phase indicated by the lattice images is consistent with the result obtained by earlier diffraction studies. A series of lattice images showing the transition from the low-temperature phase (to the high-temperature phase) has been obtained. The transformation to the high-temperature phase occurs through the penetration of the high-temperature phase into areas of the low-temperature phase. A quick motion of domain boundaries in the low-temperature phase, which is consistent with almost instantaneous rearrangements of charge ordering, has been observed. The possibility of determining the ordered arrangement of Fe²⁺ and Fe³⁺ ions directly by HREM is discussed.     

Microstructural evolution of protective La–Cr–O films studied by transmission electron microscopy

Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) were performed to study the microstructural evolution of La–Cr–O thin films deposited by radio frequency (RF)-magnetron sputtering on stainless steel substrates. Chromium L edges and oxygen K edges are analyzed to determine the valence states of the chromium and elucidate the phase evolution of the thin film. The as-deposited amorphous thin film crystallized to LaCrO₄ and finally transformed to the LaCrO₃ stable phase during annealing at 800°C. An intermediate Cr/Mn oxide layer was formed in all annealed samples. The thickness of this oxide layer stabilizes after 700°C, which indicates that the LaCrO₃ thin film plays a role in inhibiting the growth of an oxide layer on the metal surface.     

Ultrathin carbon support films for high-resolution electron microscopy of nanoparticles.

We introduce a simple preparation method for ultrathin carbon support films that is especially useful for high-resolution electron microscopy (HREM) of nanoparticles. Oxidized iron nanoparticles were used as a test sample in a demonstration of this method. The film qualities are discussed on the basis of electron-energy-loss spectroscopy (EELS) and image analysis techniques such as thickness maps and histograms. We carried out a comparison between the homemade and commercial film qualities. The relative thickness of the homemade support films was 0.6 times less than that of the commercial films, which was calculated from the EELS analysis, whereas the thicknesses of both carbon support films varied within about 3%. The percentage of the observable area was about 67 +/- 7.6% of the support film. This was about twice as large as the commercial film (32 +/- 9.3%). The HREM image of the sample prepared with our support film improved 9% in brightness and 15% in contrast compared with images obtained with the commercial support.     

Progress in aberration-corrected high-resolution transmission electron microscopy using hardware aberration correction.

The design and construction of a double-hexapole aberration corrector has made it possible to build the prototype of a spherical-aberration corrected transmission electron microscope dedicated to high-resolution imaging on the atomic scale. The corrected instrument, a Philips CM200 FEG ST, has an information limit of better than 0.13 nm, and the spherical aberration can be varied within wide limits, even to negative values. The aberration measurement and the corrector control provide instrument alignments stable enough for materials science investigations. Analysis of the contrast transfer with the possibility of tunable spherical aberration has revealed new imaging modes: high-resolution amplitude contrast, extension of the point resolution to the information limit, and enhanced image intensity modulation for negative phase contrast. In particular, through the combination of small negative spherical aberration and small overfocus, the latter mode provides the high-resolution imaging of weakly scattering atom columns, such as oxygen, in the vicinity of strongly scattering atom columns. This article reviews further lens aberration theory, the principle of aberration correction through multipole lenses, aspects for practical work, and materials science applications.     

Contrast transfer and resolution limits for sub-angstrom high-resolution transmission electron microscopy.

The optimum imaging of an object structure at the sub-angstrom length scale requires precise adjustment of the lens aberrations of a high-resolution instrument up to the fifth order. A least-squares optimization of defocus aberration C1, third-order spherical aberration C3, and fifth-order spherical aberration C5 yields two sets of aberration coefficients for strong phase contrast up to the information limit: one for variable C1 and C3, at fixed C5, another for variable C1, C3, and C5. An additional correction to the defocus aberration, dependent on object thickness, is described, which becomes important for the use of image simulation programs in predicting optimum high-resolution contrast from thin objects at the sub-angstrom scale. For instruments with a sub-angstrom information limit the ultimate structure resolution, the power to resolve adjacent atom columns in a crystalline object, depends on both the instrumental pointspread and an object pointspread due to finite width of the atomic column potentials. A simulation study on a simple double-column model yields a range for structure resolutions, dependent on the atomic scattering power, from 0.070 nm down to 0.059 nm, for a hypothetical 300-kV instrument with an information limit of 0.050 nm.     

Electron beam-induced reduction of higher oxides of the rare earths: A high-resolution electron microscopic study

Many compounds are unstable in the beam of the electron microscope or can be made so by increasing the beam intensity. Observations of beam-induced decompositions of the higher oxides of the rare earths were made under circumstances where the details of reduction in the vacuum of the microscope can be examined. The ζ and ι phases of praseodymium oxide (Pr₉O₁₆, Pr₇O₁₂) and the ι phase of terbium oxide were observed to reduce directly to the ф phase (R₂O₃), revealing the texture of intermediate stages of the reaction. The nature of the ZrO₂Sc₂O₃ system in the same intermediate composition region is also revealed and compared to the binary rare earth systems. Particular attention is paid to the appearance of composite crystals in these semicoherent phases during reaction. From these varied observations speculations on the mechanism of the decomposition are presented.     

Chemical Intergrowth of Dimeric and Trimeric Clusters in Reduced Tin Oxomolybdates: A study by high-resolution electron microscopy

By high-resolution electron microscopy, chemical intergrowth was studied of dimeric Mo₁₀O₁₆ and trimeric Mo₁₄O₂₂ clusters in reduced tin oxomolybdates within the Sn_xMo₁₀O₁₆? Sn_yMo₁₄O₂₂ system. The variety of observed intergrowth patterns includes polysynthetically twinned structures as well as a number of different superstructures. For an orthorhombic superstructure formed by alternating layers of dimeric and trimeric clusters, the lattice parameters and the Sn content were determined. The chemical reason for the amazing abundance of structures in the considered system is discussed.     

Study of structure and polymerization of epitaxially grown SiPc(OH)2 thin films by high-resolution electron microscopy

Thin films of SiPc(OH)₂ (Pc = phthalocyanine) were formed epitaxially on the (001) surface of mica by vacuum deposition and were then polymerized by heat treatment. The molecular packing of the SiPc(OH)₂ was determined by electron diffraction and high-resolution electron microscopy as triclinic${\rm P\bar 1} $ having dimensions a = 0.727, b = 1.307, c = 0.688 nm, α = 102.5, β = 104.2, and γ = 97.4°. This monomer crystal grows with its c-axis parallel to the a-axis of the substrate mica and its bc-plane parallel to the (001) surface of mica. By heat treatment at 320°C, the SiPc(OH)₂ polymerized with the c-axis of the polymer parallel to the c-axis of the monomer. At 420°C, the c-axis of the polymer became parallel to the a*-axis of the monomer (i.e., perpendicular to the film surface). From high-resolution electron microscopy of partially polymerized specimens, the polymerization was shown to start at the edges of small monomer crystals. This may be considered to be due to the volume expansion during the polymerization. © 1993 John Wiley & Sons, Inc.     

"Indirect" high-resolution transmission electron microscopy: aberration measurement and wavefunction reconstruction.

Improvements in instrumentation and image processing techniques mean that methods involving reconstruction of focal or beam-tilt series of images are now realizing the promise they have long offered. This indirect approach recovers both the phase and the modulus of the specimen exit plane wave function and can extend the interpretable resolution. However, such reconstructions require the a posteriori determination of the objective lens aberrations, including the actual beam tilt, defocus, and twofold and threefold astigmatism. In this review, we outline the theory behind exit plane wavefunction reconstruction and describe methods for the accurate and automated determination of the required coefficients of the wave aberration function. Finally, recent applications of indirect reconstruction in the structural analysis of complex oxides are presented.     

Intact and fragmented triosmium clusters on MgO: characterization by X-ray absorption spectroscopy and high-resolution transmission electron microscopy

Oxidative fragmentation of the clusters Os(3)(CO)(12) adsorbed on MgO powder was investigated by X-ray absorption spectroscopy and scanning transmission electron microscopy (STEM). Exposure of the clusters to air leads to their fragmentation, oxidation of the osmium, and formation of ensembles consisting of three Os atoms. X-ray absorption near-edge spectra demonstrate the oxidative nature of the fragmentation process. Extended X-ray absorption fine structure (EXAFS) spectra indicate an average Os-Os distance of 3.33 Angstrom and an Os-Os coordination number of 2, consistent with the formation of ensembles of three Os atoms on the support. STEM images confirm the presence of such trinuclear ensembles, and the diameters of the observed scattering centers (6.0 Angstrom) match that indicated by the EXAFS results.