[en] The local structure of chemical and polar domains and domain walls is determined directly by atomic resolution high-resolution electron microscopy. Thus the Pb, Ta and Sc atomic positions may be located in the images of very thin crystals. Furthermore the Pb cation displacements away from the ideal perovskite A-site have been measured directly for the first time. Local variations in polarization direction may be mapped directly off the images, provided certain electron optical conditions are met. The results are relevant to recent theories of polar-glass behaviour in relaxor-type complex oxide functional ceramics. 17 refs., 9 figs

[en] Characteristic differences are observed for the dielectric response and microstructures of BaTiO₃ nanoscale fine powders prepared using sol gel (SG) and steric acid gel (SAG) methods. The former exhibit a critical size below which there is no paraelectric/ferroelectric phase transition whereas BaTiO₃ prepared via the SAG route remained cubic for all conditions. Atomic resolution images of both varieties showed a high density of interesting surface steps and facets. Computer simulated images of surface structure models showed that the outer (100) surface was typically a BaO layer and that at corners and ledges the steps are typically finished with Ba+2 ions; i.e. the surfaces and steps are Ba-rich. Otherwise the surfaces were typically clean and free of amorphous layers. The relationship between the observed surfaces structures and theoretical models for size effects on the dielectric properties is discussed. (authors)

[en] Kramers-Kronig analysis for parallel electron energy loss spectroscopy (PEELS) data is developed as a software package. When used with a JEOL 4000EX high-resolution transmission electron microscope (HRTEM) operating at 100 keV this allows us to obtain the dielectric function of relatively wide band gap materials with an energy resolution of approx 1.4 eV. The imaginary part of the dielectric function allows the magnitude of the band gap to be determined as well as the joint-density-of-states function. Routines for obtaining three variations of the joint-density of states function, which may be used to predict the optical and dielectric response for angle-resolved or angle-integration scattering geometries are also described. Applications are presented for diamond, aluminum nitride (AlN), quartz (SiO₂) and sapphire (Al₂O₃). The results are compared with values of the band gap and density of states results for these materials obtained with other techniques. (authors)

[en] The analogy between carbon nanotubes and synthetic imogolite, an aluminosilicate of stoichiometry (OH)₃Al₂O₃SiOH, having a tubular structure of external diameter 2.4nm and internal diameter 0.9nm, is examined. High resolution transmission electron microscopy, computer-simulations and image-matching and electron diffraction are used to examine the tube structure. Some interesting new results are obtained, concerning the various states of aggregation of the imogolite tubes, ranging from randomly oriented single tubes to essentially close-packed arrays. Both longitudinal and cross-sectional images were obtained. These confirm directly the tubular structure deduced by X-ray structure analysis. Some possible applications of imogolite as new materials are discussed. The various imogolite textures pose challenging problems for solid state physicists, concerning e.g. the transport and optical properties of such fibre bundles. (authors)

[en] Chemical domain textures for lead scandium tantalate (PST) are modelled using Monte Carlo (MCS) and next-nearest-neighbour Ising (NNNI) models. A wide range of degrees of short- and long-range ordering of the (Ta,Sc) atoms occur in ceramic specimens, depending on processing routes. The simulations help to understand and quantify the chemical domain textures, chemical domain wall configurations and other chemical defects which may occur in certain relaxor-type perovskite-type oxides. The results are compared to dark-field transmission electron microscopic observations. Some new types of small defects were discovered. These are described and classified. The results provide a first step towards the development of a microscopic statistical physics framework for analytical theories of the dielectric response of relaxor-type ceramics, where the frequency and temperature variation of the permittivity are due essentially to dipolar-type fluctuations on nanometer scales. 25 refs., 8 figs

[en] Both the mass density (1.37 kgm/m³) and sp²+sp³ bonding fraction (0.15) were determined for an unusual nanoporous amorphous carbon consisting of curved single graphitic sheets. A combination of high-resolution transmission electron microscopy (HRTEM) and parallel electron energy loss spectroscopy (PEELS) was used. The values of these two parameters provide important constraints for the determination of the structure of this relatively low density variety of nanoporous carbon. The results are relevant also in the search for negatively-curved Schwarzite-related carbon structures. New date are also presented for highly-oriented pyrollytic graphite (HOPG), chemically vapour deposited (CVD) diamond, C₆₀, glassy carbon (GC) and evaporated amorphous carbon (EAC); these are compared with the results for NAC. Kramers-Kronig analysis (KKA) of the low-loss PEELS data shows that the band gaps of both NAC and EAC are collapsed relative to that of CVD diamond. 18 refs., 2 tabs., 3 figs

[en] High-resolution transmission electron microscopy, electron diffraction and parallel electron energy loss spectroscopy are used to analyse nanocrystalline diamond powder. Grains of diameter in the range 2- 10 nm were found aggregated together; the grain boundaries were essentially a grossly disordered (amorphous) intergranular phase. Analysis of the plasmon loss-function indicated mass density of 3.30 g/cm³, compared with 3.51 g/cm³ for a chemically-vapour-deposited diamond. The core-loss spectra showed virtually pure sp³ bonding overall although some exposed surfaces were coated with two or three graphitic layers. Two peaks were observed in the low energy loss-function, one at 34 eV was characteristic of the volume plasmon typically observed in crystalline diamond, a second peak at ∼ 23 eV for larger grains, shifted to lower energies as the particle size decreased (to 19.5 eV for 2.8 nm diameter) and at the same time it increased in intensity, becoming stronger than the volume plasmon for a 2.8 nm crystal. these results are interpreted using theoretical results for surface plasmons in small spherical particles. 19 refs., 1 tab., 5 figs

[en] (0001) and (1120) polar surface facetting of sapphire and ruby is observed directly at atomic resolution under electron irradiation. For atomically clean surfaces, monolayers of spinel-type Al₃O₄ structure have been identified using computer modelling and image-matching. For surfaces precoated with thin amorphous layers there is no irradiation-induced rearrangement and simple metal atom surface terminations have been identified. The results are discussed in terms of alternative surface depolarization mechanisms, which should be relevant to electronic, catalytic and ceramic applications of alumina

[en] Short communication

[en] A Kramers-Kronig analysis program is developed as a custom function for the GATAN parallel electron energy loss spectroscopy (PEELS) software package EL/P. When used with a JEOL 4000EX high-resolution transmission electron microscope this program allows to measure the dielectric functions of materials with an energy resolution of approx 1.4eV. The imaginary part of the dielectric function is particularly useful, since it allows the magnitude of the band gap to be determined for relatively wide-gap materials. More importantly, changes in the gap may be monitored at high spatial resolution, when used in conjunction with the HRTEM images. The principles of the method are described and applications are presented for Type-1a gem quality diamond, before and after neutron irradiation. The former shows a band gap of about 5.8 eV, as expected, whereas for the latter the gap appears to be effectively collapsed. The core-loss spectra confirm that Type-1a diamond has pure sp³ tetrahedral bonding, whereas the neutron irradiated diamond has mixed sp²/sp³ bonding. Analysis of the low-loss spectra for the neutron-irradiated specimen yielded density 1.6 g/cm³, approximately half that of diamond. 10 refs., 2 figs