J. L. Hutchison

Electron microscopy of ferroelectric bismuth oxides containing perovskite layers

Ferroelectric bismuth oxides of the general formula (Bi2O2)2+(An-1BnO3n+1)2- (A = Ba, etc., B = Ti or other transition metal) have been examined by high-resolution lattice imaging electron microscopy. The lattice images show dark bands at the positions of the Bi20 2 layers, with n -1 lines between them due to the layers of the perovskite A cations or, in favourable circumstances, the fully resolved 0.4 nm square perovskite grid. Dislocations and domain boundaries have been imaged for the first time in ferroelectric crystals. The structure of the dislocations and domain walls is discussed in the light of the microstructural evidence.

On voidites: a high-resolution transmission electron microscopic study of faceted void-like defects in natural diamonds

In natural diamonds of optical classification type la , nitrogen is the major identified impurity and is distributed mainly in point defects known as A defects (probably a pair of nitrogen atoms substituting for a pair of adjacent carbon atoms) and B defects (probably four substituted nitrogen atoms tetrahedrally surrounding a carbon vacancy), and also in the electron-microscopically visible platelet precipitates on {100}. This paper is concerned with other electron-microscopically detectable defects, discovered by R. F. Stephenson (Ph.D. thesis, University of Reading (1977)), that lie in {100} planes in circumstances strongly suggesting that they result from the decomposition of platelets. High-resolution electron microscopy shows these defects to be {111}-faceted cavities. They behave as pure phase-contrast objects whose interior density does not exceed about one-third that of the diamond matrix: we call them ‘voidites’. The experimental background to voidite observation is reviewed, including electron-microscopic measurements on normal {100} platelets and models of their structure, and the optical, X-ray diffraction and cathodoluminescence evidence for unusually large platelets whose presence, together with a relative richness in B defects, indicates an environment in which voidites are likely to be discovered. Almost all observed voidites are confined to sheets strictly parallel to {100}. Some voidite sheets occur in ‘ partial platelets’, where they replace part of the original area of normal platelet. Other voidite sheets occur within dislocation loops whose size and shape are similar to those of the peripheries of normal platelets in the specimen. Voidites occur in a wide range of sizes. The largest equiaxed voidites observed measure about 10 nm between opposite {111} facets, and the smallest resolved about 0.5 nm. Many voidites are elongated in one of the <110> directions in the plane of the voidite sheet: the most highly elongated voidites seen approach 100 nm in length, with diameters of a few nanometres. Variations in size, shape and number density of voidites, together with many other characteristics relevant to the microscopic processes of voidite formation, are discussed in detailed descriptions of about 40 voidite sheets occurring in partial platelets and dislocation loops in two diamond specimens. One specimen was free from both grown-in dislocations and dislocations associated with plastic deformation. It contained zones of highly elongated platelets and it appeared that transformation of a platelet into a voidite sheet surrounded by a dislocation loop was triggered by the mutual very close approach of platelets. The second voidite-containing specimen had suffered plastic deformation at some stage in its history, but did not exhibit direct evidence that glide dislocations had triggered the transformation. The Burgers vectors of 24 dislocation loops enclosing voidite sheets in the second specimen were determined. Twelve were of normal ½<110> type having a component ½a0 normal to the voidite sheet, and twelve were non-primitive, the Burgers vector being a0 normal to the voidite sheet (a0 is the diamond face-centred cubic (fcc) unit cell edge). The volumes of over 2000 individual voidites, representing all or major parts of 12 voidite sheets, have been measured. Values found for the ratio ∑V/Aa0 (where ∑V is the aggregate voidite volume in a sheet area A) averaged about unity for 9 sheets of generally similar, voidite-rich appearance. Other sheets are poorer in voidites of measurable dimensions: the ratios for two such sheets averaged 0.25. In the concluding analysis, a reaction involving A and B point defects is proposed for the production of platelets. Other reactions including voidites (but no dislocations) are suggested in which both platelet production and elimination might occur. For the dominating reaction, when a platelet is replaced by a voidite sheet surrounded by an interstitial dislocation loop, models are developed for the cases when the Burgers vector component perpendicular to the loop is either a0 or ½a0, with the assumption that the platelet nitrogen is dispersed partly into B defects and partly into the voidites. The predicted values of ∑V/Aa0 come out as about unity and as 0.25 (or lower) for the larger and smaller Burgers vectors, respectively.

The internal structure of nephrite: experimental and computational evidence for the coexistence of multiple-chain silicates within an amphibole host

Ultra-structural variations in samples of nephrite jade have been elucidated by a combination of high resolution electron microscopy, which explores local structure in a direct, ‘real space’ manner, and computational procedures using the so-called ‘multislice’ approach which enables the image generated under a given set of electron-optical conditions to be calculated as a function both of unit cell content and of specimen thickness. Microanalyses by electron-stimulated X-ray emission and by optical diffractometry (of micrographs) were also used. Planar faults on (010) occur frequently in nephrite. These are of several distinct kinds, all of which have been characterized. The host amphibole, consisting of double chains of linked SiO4 tetrahedra, is shown frequently to accommodate triplechain lamellae which are coherently attached to the double-chain matrix on (010) planes. These triple-chain faults may occur in isolated fashion, but sometimes are arranged recurrently, in a disordered manner, within the amphibole host. The nephrite may also accommodate extended regions of a new, triple-chain mineral structure, the maximum observed width of this coexistent phase being ca. 340 A. Other planar faults, composed of regions with chain widths ranging from one to six SiO4 tetrahedra, have been detected and fully identified on the basis of the correspondence between theoretically calculated and observed images. Detailed structural drawings for the continuous planar faults, as well as others that are described, are given. It is shown that edge-sharing of tetrahedra probably occurs close to the termini of certain types of discontinuous fault and that, in other cases of defect termination, screw-type dislocations may be incorporated to preserve the strain-free, structural regularity of the host. It has not yet proved possible, with currently available in situ X-ray microanalytical techniques, to assign chemical compositions to the new structural types that have been brought to light by this study.

Dislocations and Related Defects in Niobium Oxide Structures

Lattice images of the niobium oxides, structures based on the linkage of octahedral groups in continuous networks, occasionally contain features recognizable as dislocations. Since lattice imaging enables the micro-structure to be resolved in great detail, at the level of local structural organization, it is possible to determine the configuration, and also to infer the chemical composition, of dislocated areas. By treating the niobium oxide ‘block’ structures as superstructures of the ReO3 (DO9) type, the topology of dislocations can be expressed by relations between the insertion (or deletion) of one or more half-planes of cations, or of oxygen atoms only, changes in the number of crystallographic shear plane interfaces between blocks or columns, changes in (idealized) dimensions and any requisite distortion in the third dimension. Mapping the structure around a dislocation, from the lattice image, is directly equivalent to plotting the Burgers’ circuit. In this way, the precise nature of a dislocating perturbation and its implications for the local chemical composition of the crystal can be directly identified. The method is exemplified by analysis of dislocations and of related extended defects of several types, associated with twinning phenomena, semicoherent intergrowth between different ReO3-type superstructures and arrays building up a low angle boundary. The essential features of the analysis are not restricted to structures of the niobium oxide type, but can be extended to other types of polyhedron networks.

Linear and planar defects in wollastonite

Wollastonite from various sources has been examined by high-resolution electron microscopy and lattice images are presented which show a wide range of defect structures. Although these images do not show a one-to-one correspondence with the structure, it is shown that they can be used in the observation and characterisation of planar and linear defects.

Electron microscopy of the niobium oxides. Part II. Multiple phases in the system Nb2O5+ MgF2

The formation of mixed phases in the system MgF2+ Nb2O5 has been examined, on the grounds that similarity in ionic radii between Mg and Ti, and between F and O, should lead to the analogues of the known titanium–niobium block structure oxides. Electron diffraction and direct electron microscope lattice imaging methods show that, in addition to the compound already described as MgNb14O35F2, with the (5 × 3)∞ structure, phases iso-structural with H-Nb2O5, N-Nb2O5, TiNb24O62, and Ti2Nb10O29 are formed. By compensatory substitution, each of these structures represents a solid solution series and mixtures with compositions on the MgF2–Nb2O5 tie line yield multiphase products. Regular intergrowth structures may be formed between the basic structural types.

Preparation of Ag2O crystallites within phospholipid vesicles and their use in nucleation studies

Ag2O crystallites have been prepared within phosphatidylcholine vesicles and characterised by high resolution electron microscopy; the potential of the approach for nucleation studies generally is described.

Discovery of new types of chain silicates by high resolution electron microscopy

The existence of infinite one-dimensional structures consisting of linked triple-, quadruple-, and sextuple-chains of SiO4 tetrahedra has been unambiguously established by electron microscopy.