Crystallographic shear defect in molybdenum oxides: Structure and TEM of molybdenum sub‐oxides Mo18O52 and Mo8O23

Molybdenum trioxide and molybdenum sub‐oxides are of great interests in catalysis due to their utilities as model system to elucidate the correlations between the structure and the catalytic performance. The suboxides are usually an intermediate phase during catalytic reaction in which the lattice oxygen is involved. We show the identification of the two common molybdenum sub‐oxides Mo₁₈O₅₂ and Mo₈O₂₃, derived from MoO₃ by crystallographic shearing (CS), by means of electron diffraction and High‐Resolution Transmission Electron Microscopy (HRTEM) in combination with image simulation. The coincidence of simulated electron diffraction patterns and high‐resolution images with the experimental ones indicates the feasibility of CS structure determination by these techniques.     

Crystallographic defects in epitaxial layers of cadmium sulphide

Thin epitaxial films of CdS deposited in high vacuum on ionic single crystal substrates have been studied by transmission electron microscopy. The (100), (110) and (111) faces of NaCl and the (111) face of BaF₂ were used as substrate surfaces. Both cubic sphalerite and hexagonal wurtzite structure films have been produced. The orientations of the sphalerite structure films were (100) and (110) and were produced on substrate faces having the same two orientations. The wurtzite structure films were in (0001) orientation and grew on (111) oriented substrate faces. For a fixed rate of deposition both the number and type of defects found in the films appear to be dependent upon the growth temperature and the crystal structure. Annealing the films at a high temperature has been tested as a means for reducing their defect content and the effect is very different for the two crystal structures. A reduction in the defect content of the wurtzite structure films is induced but no change in the crystal structure occurs. In contrast to this, the sphalerite structure films undergo a progressive phase transformation to the wurtzite stucture while at the same time losing a high proportion of their defects.     

Study of the structure of alkali–silica reaction gel by high-resolution NMR spectroscopy

The alkali–silica reaction is a deleterious chemical process that can occur in concrete. The product of the reaction is an amorphous silicate material with gel characteristics, whose high expansion properties may cause cracking in the matrix and in the discrete aggregate of particles, leading to severe deterioration of the concrete structure. Structural information of this gel at the atomic scale can provide critical information on how to develop appropriate repair of the affected structure. Samples of this gel, produced under in-service conditions in a large concrete dam, were studied by ²⁹Si and ²³Na high-resolution nuclear magnetic resonance spectroscopy, triple quantum magic angle spinning ²³Na nuclear magnetic resonance, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The short-range atomic structure of the compound was determined as an amorphous potassium–hydroxide–silicate glass, with a Q³-like dominant silicate connectivity, having a silicate speciation highly disproportioned when compared with potassium–silicate glasses with the same K₂O content. Sodium ions are mostly segregated from the bulk amorphous silicate network, forming crystal domains attributed to the trona compound (sodium sesquicarbonate, Na₂CO₃ · NaHCO₃ · 2H₂O). The structural picture at atomic scale obtained in this study gives support for double-layer models of the expansive properties of the alkali–silica reaction gel.     

Structural aspects of volume nucleation in silicate glasses

An interrelationship between parameters of short and intermediate range order in silicate glasses and the tendency to nucleate homogeneously in the volume is tested. Changes in the average coordination number and metal-oxygen distance of network modifying cations as well as changes in the concentration of constitutive silica tetrahedra accompanied with the crystallization of 18 stoichiometric glass compositions into their crystalline analogs are determined. The intermediate range structure of the glasses is investigated by configurational entropy and flow birefringence. The changes in structural parameters are analyzed in terms of the reduced glass transition temperature T_rg, which is negatively correlated with the maximal rate of volume nucleation. The results indicate that the short-range structure in stoichiometric glasses is, in general, very similar to the corresponding crystal structure but independent of the T_rg-scale and for this reason independent of nucleation properties. In contrast to the short range of the glass structure, birefringence induced by a forced flow above the glass transition temperature and configurational entropy are positively correlated with increasing T_rg. The results indicate increased structural order in the intermediate range for melts with a high supercool limit (T_rg < 0.58). It is concluded that this order phenomena may promote nucleation events and may help to explain the tendency to volume nucleation of silicate glasses with T_rg < 0.58.     

Transmission Electron Microscopical Studies of the Layered Structure of the Ternary Semiconductor CuIn5Se8

The structure of the off‐stoichiometric In‐rich ternary phase CuIn₅Se₈ was studied by means of electron diffraction and high‐resolution electron microscopy. The compound shows a layered structure with a 7‐layer stacking sequence of closed‐packed planes, which contains both cubic and hexagonal stacking of Se atoms. The studied CuIn₅Se₈ bulk crystal is known as the β‐phase of this compound.     

Über die Kristallstrukturdifferenzen der MeSO4 · nH2O-Salztypen

On the basis of the systematized data on the crystal structure of salts of the MeSO₄ · nH₂O type, where Me = Mg²⁺, Ni²⁺, Zn²⁺, Co²⁺, Fe²⁺, Cu²⁺ and n = 0–7, it is established that the differences in the structures of given chemical type metal sulphate crystal hydrates are determined by the nature of the metal ion. The differences in the crystal structure of the heptahydrate, as well as the existence or non-existence of crystal hydrates of some of the ions considered are explained with the electron configuration of the metal ions. The deformation trend of the octahedral coordination of the metal ions considered increases in the following way:      

Rasterelektronenmikroskopische Kontrastuntersuchungen an Germanium-Einkristallfolien

Statements made after scanning electron microscope contrast investigations on the geometrical and physical properties of thin, foil-like crystal regions strongly depend on the utilized interaction between electron probe and specimen. At the example of germanium single crystal foils of a thickness of only a few hundred Å in thinned regions the image contrasts resulting from secondary, backscattered, and absorbed electrons within an accelerating voltage range of between 15 and 45 kV are discussed. This is done based on the present knowledge of the ejecting mechanisms of the three interactions. It is not yet determined whether by means of adequate calibration methods a technique for evaluation of the thickness of such specimens can be developed from this.     

The effect of polyacrylamide on the crystallization of calcium carbonate: Synthesis of aragonite single-crystal nanorods and hollow vatarite hexagons

CaCO₃ nanorods were synthesized via a facile solution route by polymer-controlled crystallization in the presence of polyacrylamide (PAM). The morphology, size and crystal structure were characterized by means of scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). The results suggest that the as-synthesized product was CaCO₃ (aragonite) nanorods with diameter ca. 50 nm and length ca. 1 μm. Selected area electron diffraction (SAED) pattern shows the single-crystal nature of CaCO₃ nanorods. The reaction time, temperature, pH and reactant concentration were systemically investigated. With the increase in the reaction time, hollow vaterite hexagonal disks can be obtained.     

Electron Optical Investigation of Growth-Induced Inhomogeneities of Synthetic Hg2Cl2 Monocrystals

The submicroscopic structure of synthetic Hg₂Cl₂ monocrystals was investigated by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), indirect imaging of surfaces (Pt/C replica technique) as well as by application of supplementary methods. As already reported by NEUMANN et al., TEM-images show a periodic stripelike contrast whose period varies from 0.1 m̈m to 0.2 m̈m. It proved that the stripelike contrast arises from a periodic chemical inhomogeneity of the synthetic crystals. The chemical inhomogeneity probably consists in periodic deviations from the stoichiometric composition of Hg₂Cl₂. By reason of the orientation on {110}-, (001)-, and {114} faces a reconstruction of arrangement of the structures in the crystals is carried out. According to that, the stripes have to be interpreted as intersection lines of 2-dimensional formations with the corresponding faces.     

Si modified Ge10 cluster: the preparation and characterization of single crystal Na4(Ge,Si)9O20

A Si modified Ge₁₀ cluster with structure Na₄(Ge,Si)₉O₂₀ (denoted as HUT‐1) was synthesized by hydrothermal synthesis at 160 °C with a sodium silicate source. The compound was characterized by single crystal, powder X‐ray diffraction and TGA‐DSC analysis. HUT‐1 crystallizes in space group I4₁ (80) with calculated unit cell (a=14.966(5) Å, c=7.343(2) Å, V=1644.8(9) ų), which has the same structure as Na₄Ge₉O₂₀. HUT‐1 has a high Si/Ge ratio with an approximate formula of Na₄Ge_7.68Si_1.32O₂₀. Single crystal X‐ray structure refinements together with results from X‐ray powder diffraction (XRPD) confirm the occupancy of Si at two tetrahedral sites. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Characterization of Y2Pd14B5

Y₂Pd₁₄B₅ is the major phase in as‐cast and annealed multiphase alloys with nominal compositions near YPd₅B₃C_0.3. Its crystal structure determined the first time here by single crystal X‐ray diffraction is body‐centered tetragonal (space group I4₁/amd). Transmission electron microscopy (TEM) reveals that in as‐cast specimens the tetragonal phase has a modulated structure and is oriented intergrown with a face‐centered cubic phase of similar composition, namely YPd₇B₂. According to Rietveld analyses of the multiphase system the structure of this phase can be well‐described by space group Fm m. Annealing the sample for 150 hrs at 973 K results in a coarsening and enrichment of the tetragonal phase as well as a disappearance of the modulations allowing a detailed structure analysis by single crystal diffractometry.     

Structure investigations of the Ge-GeO2 system by high-resolution electron microscopy

The crystal structure of thermally oxidized Ge was investigated by high-resolution electron microscopy (HREM), mainly the interface Ge/oxide. Under special conditions the reaction Ge + O₂ → GeO₂ which takes place at (111) surface planes leads to suitable thin crystal regions. The GeO₂ occurs normally as amorphous films on the crystal surface. Furthermore, hexagonal GeO₂ can grow at the interface Ge/oxide by a topotaxial reaction; the orientation relation between these two lattices was ascertained. Intensive electron irradiation was used to initiate and to observe structure changes in boundary regions.     

Structure and photoluminescence of amorphous silicate composites containing ZnO particles synthesized from layered sodium silicate

This study has been conducted to synthesize ZnO/silicate composites from layered sodium silicate (δ-Na₂Si₂O₅). For this purpose, various techniques have been employed to characterize the composites. X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR) indicate that ZnO/silicate composites are amorphous. Scanning electron microscope equipped with energy dispersive X-ray (SEM-EDX) was used to estimate the chemical compositions of the composites. Photoluminescence (PL) spectra of ZnO/silicate composites present a significant blue-shift of peak position and increasing intensity. The size of ZnO embedded in silicates was estimated to be 10–20 nm by transmission electron microscope (TEM). Thermogravimetric and differential thermal analyses (TG–DTA) suggest that the ZnO/silicate composites are different from the ZnO and layered sodium silicate. All these results confirmed that the amorphous ZnO/silicate composites formed and the photoluminescent property of ZnO embedded in the silicate was obviously improved in comparison with the uncovered ZnO.     

The crystal structure of Zr2Se reinvestigated by electron crystallography and X-ray powder diffraction

The metal-rich compound Zr₂Se is of particular interest for electron crystallography, since it was one of the first examples that proved that heavy-atom structures can be solved via quasi-automatic direct methods from selected area electron diffraction intensities [1]. For this reason, Zr₂Se has been chosen as a model to discuss the possibilities and the limits of the quasi-kinematical approach that has been successfully used to determine this and related structures from high-resolution electron microscopy (HREM) images and selected area electron diffraction. In order to quantify the achievable accuracy of the electron crystallography techniques used, the corresponding structures are compared with results from structural analysis with X-ray powder data and with a model received from first-principles calculations. The latter structure was chosen in this study as a reference, since the calculations do not depend on experimental parameters. Analysis of the obtained result from electron diffraction structural analysis (EDSA) shows that the structural model is, on average, only off by 0.08 Å, despite the investigated crystal having an effective thickness of 286 Å. The corresponding result from Rietveld refinement with X-ray powder data agrees to within 0.04 Å with the structure from calculation and within 0.03 Å with the result from an earlier single crystal X-ray study [2].     

Some characteristic features of formation of composite material based on KDP single crystal with incorporated Al2O3·nH2O nanoparticles

Composite material based on KDP (KH₂PO₄) crystal matrix with incorporated aluminum oxyhydroxide Al₂O₃·nH₂O nanoparticles is obtained and peculiarities of the formation of KDP:Al₂O₃·nH₂O composite structure are studied by selective etching, optical and scanning electron microscopy. Influence of the nanoparticles on the formation of defect subsystem is analyzed. The obtained material is shown to have a zonal structure with a period independent of the concentration of nanoparticles. By means of FTIR‐spectroscopy, interaction of nanoparticles with KH₂PO₄ solution is studied. A model of the capture of nanoparticles by the {100} KDP crystal face is proposed.     

A Polymorph Crystal Structure of Hexaphenyl Observed in Thin Films

Highly oriented thin films of hexaphenyl — which are used in organic opto‐electronic applications — are characterised in terms of their crystal structures. Two different crystal structures of hexaphenyl (C₃₆H₂₆) are observed when the films are prepared by physical vapour deposition at various substrate temperatures. If the substrate is kept at room temperature, hexaphenyl crystallises within a structure which is already known from single crystal investigations. However, when the thin films are grown at a substrate temperature of 160°C a new crystalline phase appears. This structure was characterised by X‐ray and transmission electron diffraction. Due to the strong preferred orientation of the crystallites within the thin films, the lattice constants as well as main features of the new crystal structure could be determined. The lattice is indexed as monoclinic with: a = 7.98Å, b = 5.54Å, c = 27.64Å and β = 99.8°. The new crystal structure has high similarity to the already known crystal structure: Both structures are built by layers of hexaphenyl molecules, within one layer the aromatic planes of the hexaphenyl molecules are packed in a herringbone pattern. The characteristic feature of the new structure is that the long axes of the hexaphenyl molecules are arranged absolutely perpendicular to the layers, whereas, within the already known structure the long axes show an tilt angle of 17° to the layer normal direction.     

Preparation and characterization of CdGeAs2 crystal by modified vertical Bridgman method

A large, crack-free CdGeAs₂ single crystal measuring 15 mm in diameter and 45 mm in length was grown in a vertical three-zone tubular furnace by a modified vertical Bridgman method, i.e. quasi-seed technique with small temperature gradient and descending quartz ampoule. High-purity, single phase CdGeAs₂ polycrystallite for crystal growth was synthesized using a rocking furnace with temperature oscillation techniques. Various measuring means, including X-ray diffractometer(XRD), Fourier transform infrared spectroscopy(FTIR), and Field emission scanning electron microscope(FE-SEM) were adopted to characterize the as-grown crystal. It is found that the cleavage plane of the as-grown crystal is {1 0 1} face; the crystal is integrated in structure and crystallized well; etch pits in the shape of pentagon on (1 1 2) face have been observed for the first time using the new preferential etchant we prepared. All these results encouragingly indicate that the modified vertical Bridgman method is a convenient and effective way for high quality CdGeAs₂ crystal growth.     

Electron microscopic study on nucleation and growth of silver particles in a thin glass film

Nucleation and crystal growth of silver particles in a thin glass film of Li₂O·2.6SiO₂ glass containing 0.1 wt% of Ag₂O were observed by transmission electron microscopy and studied by an electron diffraction technique. Anomalies of the image contrast which appeared in the heat-treated specimens were explained to be caused by phase separation of the glass. Nucleation of silver particles was found to occur on the surface of the phase-separated silica-rich droplets. As the silver particles grew, their shape changed into that of a regular cube which was covered with a thin diffusion layer of silver ions about 50 Å in thickness. The presence of this thin layer and the cubic shape suggest that the growth of silver particles proceeds by a layer-by-layer growth mechanism.     

Synthesis and characterization of Bi3.15Nd0.85Ti3O12 nanotube arrays

Nd-substituted bismuth titanate (Bi_3.15Nd_0.85Ti₃O₁₂, BNT) nanotube arrays are fabricated by means of a sol–gel method utilizing porous anodic aluminum oxide (AAO) template. The morphologies and structures have been determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The diameter and length of these nanotubes are about 200 nm and 60 μm, respectively, and their wall thickness is about 30 nm. The average grain size is around 40 nm. XRD data show that the BNT nanotubes possess bismuth-layered perovskite structure. High-resolution electron microscopy (HRTEM) image demonstrates that the BNT nanotubes are polycrystalline. Polarization–electric field (P–E) response curves of BNT nanotube arrays were measured, and a size induced polarization reduction phenomenon is observed.     

Optical properties of Sr3TaGa3Si2O14 single crystal

The specific rotation ρ of strontium tantalum gallium silicate Sr₃TaGa₃Si₂O₁₄ (STGS) piezoelectric single crystal was determined from 350 to 850 nm by measuring the optical transmission between parallel polarisers in Z direction. It is shown that Sr₃TaGa₃Si₂O₁₄ has quite large a value of ρ which is a little smaller than that of strontium niobium gallium silicate Sr₃NbGa₃Si₂O₁₄ (SNGS). The crystal with ordered structure which is isostructural to calcium gallium germanate Ca₃Ga₂Ge₄O₁₄ (CGG) was grown by Czochraiski technique. And its birefringence was also determined. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)