L. A. Bursill

The Raman spectrum of nanocrystalline diamond

Abstract Nanometre sized diamond powder has been purified by centrifugation to remove contamination from sp2 bonded carbon. The purified powder has been characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The EELS spectra confirmed the absence of sp2 bonded carbon and showed strong contributions from surface plasmons. Strong relatively sharp peaks are observed in the Raman spectra at 500, 1140, 1132 and 1630 cm −1 . By comparing the Raman spectra of the nanodiamond clusters with that of amorphized diamond and with calculations of the vibrational density of states we are able to suggest the origin of features in the vibrational spectrum from nanocrystalline diamond.

Size effects on ferroelectricity of ultrafine particles of PbTiO3

High resolution transmission electron microscopy is used to image the nanostructure of ultrafine ferroelectric lead titanate particles ranging from 20 to 2000 nm in diameter. The crystal structure, surface morphology, domain–wall structure, as well as surface reconstruction under a 400 KeV electron beam are studied. High resolution images and selected area diffraction patterns showed that all the particles had tetragonal structure; the c/a ratio and domain size both decreased with decreasing particle size and the particles became monodomain when their diameter was less than 20 nm. A domain wall width of 14 A was deduced from strain contrast shown by 90° domain walls. There is no evidence of amorphous surface layers; however surface reconstruction of lead titanate particles under the electron beam irradiation may produce small particles identified as face-centred PbO. The relationship between the physical properties and the observed nanostructures is discussed.

Imogolite: An aluminosilicate nanotube material

Abstract The analogy between carbon nanotubes and synthetic imogolite, an aluminosicate of stoichiometry (OH)3Al2O3SiOH, having a tubular structure of external diameter 2.4 nm and internal diameter 0.9 nm, is examined. High-resolution transmission electron microscopy, 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. Some possible applications of imogolite as new materials are discussed. The various imogolite textures pose challenging problems for solid-state physicists, concerning for example the transport and optical properties of such fibre bundles.

Chaotic states observed in strontium barium niobate

Abstract High-resolution dark-field images of strontium barium niobate (BaSrNb4O12) reveal a hierarchy of chaotic states imposed upon the essentially tetragonal-tungsten-bronze framework structure. Thus, 180° ferroelectric domains were observed, at room temperature, having spatial extent < 500 A. Within these, 90° ferroelastic domain textures were discovered, revealing orthorhombic symmetry for strontium barium niobate. Within each domain an incommensurate superlattice was found, which contains intergrowth of two different orthorhombic structures. Finally, high-resolution dark-field images show fluctuations in the occupancy of Ba and Sr sites along [001] tunnels of strontium barium niobate, since their average occupancy is 5/6. The contributions these various chaotic states may make to the thermodynamic stability and singular optical properties of strontium barium niobate are discussed.

Heavy-ion irradiation tracks in zircon

Abstract Heavy-ion irradiation (14 MeVu−1 Pb ions) of zircon crystals gives rise to linear latent tracks of 80 A diameter and length 140 μm. Direct observation of the track core, by high-voltage high-resolution electron microscopy at atomic resolution, reveals a core having roughly circular cross-section, with some facetting of the core/matrix interface on {101} planes of zircon. The core diameter appears quite uniform. Conventional transmission electron microscopy (bright- and dark-field imaging) reveals an elastic strain field extending for a short distance into the zircon matrix. This appears to drop off more rapidly with distance, say 1/R2, than do dislocation strain fields (∼ 1/R). Analysis of the various contrast mechanisms yields the result that the core is essentially amorphous. Our observations confirm directly the conclusions of Fleischer, Price and Walker, based on track etching and electrical conductivity measurements, that the irradiation damage is confined to a 50–100 A core region of atomic...

Diamond nanocrystals formed by direct implantation of fused silica with carbon

We report synthesis of diamond nanocrystals directly from carbon atoms embedded into fused silica by ion implantation followed by thermal annealing. The production of the diamond nanocrystals and other carbon phases is investigated as a function of ion dose, annealing time, and annealing environment. We observe that the diamond nanocrystals are formed only when the samples are annealed in forming gas (4% H in Ar). Transmission electron microscopy studies show that the nanocrystals range in size from 5 to 40 nm, depending on dose, and are embedded at a depth of only 140 nm below the implanted surface, whereas the original implantation depth was 1450 nm. The bonding in these nanocrystals depends strongly on cluster size, with the smaller clusters predominantly aggregating into cubic diamond structure. The larger clusters, on the other hand, consist of other forms of carbon such as i-carbon and n-diamond and tend to be more defective. This leads to a model for the formation of these clusters which is based o...

Experimental and calculated images of planar defects at high resolution

Abstract Calculations of the contrast in n-beam images of isolated edge-on crystallo-graphic shear planes (CSP) have been carried out. The multi-slice method in combination with periodic continuation was used, thereby overcoming the resolution limitations inherent in the column approximation. The effects of crystal thickness, crystal tilt, CSP shear vector, defoeus, spherical aberration and objective aperture have been investigated and the results confirmed by comparison with experimental focal-series images. The applicability of the projected-charge-density approximation for the intuitive interpretation of image contrast at a resolution of about 4 A has been demonstrated for both (132) and (210) CSPs in rutile. The latter provide examples of metal-rich and metal-deficient faults, respectively.

Chemical twinning and ferroelectric domains in LiTaO3

Abstract Ferroelectric twinning, with {0112} and {0114} twin composition planes, has been induced in lithium tantalate by partial exchange of Li+ by Ag+. This process leads to new types of ferroelectric domains, which involve rotations of the polarization vector by 66°, 104°, 75.4° and 104.6°. The domain walls are chemical in origin and very much more stable than the well-known 180°-ferroelectric domains in pure LiTaO3. The structure of the domain walls is clarified by a crystallographic analysis of the possible structures and interpretation of high-resolution electron microscope images and selected-area diffraction patterns. Non-polar boundaries, or stacking faults, and recurrent twin dislocations, or ‘swinging superlattice structures’, were also observed and analysed.

Observation and analysis of nanodomain textures in the dielectric relaxor lead magnesium niobate

Abstract High-resolution (0.2 nm) images are used to locate chemical domains occurring with length scales of 1–5 nm in the dielectric relaxor lead magnesium niobate (PMN). The experimental HRTEM images are analysed using computer simulations and image matching in order to clarify and characterize the nature of the chemical ordering. Madelung electrostatic energy calculations are used to rank a set of structural models for possible ordered and disordered distributions of Nb and Mg over the B sites of perovskite ABO3. Next, the chemical domain textures are modelled using next-nearest-neighbour Ising (NNNI) models and Monte Carlo methods. These simulations allow us to understand and quantify the local structures of clusters of Nb and Mg atoms and how these interact to form chemical domain walls and other configurations including charged chemical defects. This results in a preferred model for the B-site distribution (the extended NNNI model), which is used for image simulations. Both HRTEM many-beam bright- and dark-field and single-beam dark-field TEM images are obtained and compared with the experimental images. The final result is a realistic atomic model for the Nb, Mg distribution of PMN.

Twinning dislocations in haematite iron ore

Abstract Direct observation of twinning dislocations at rhombohedral twin interfaces in naturally deformed haematite ore is reported. An analysis of the coordination polyhedra produced by the operation of the observed twinning elements across {1012} composition planes leads to a simple description of the interface structure whereby Fo atoms occupy tetrahedrally coordinated sites. Simple displacive atomic movements allow for rapid propagation of the twinning dislocations. The structural results are also relevant to mechanical twinning of sapphire and other structures derived from hexagonal close-packing, including hexagonal metals and alloys.