L.A. Bursill

Temperature dependence of Raman scattering in single crystal GaN films

Micro-Raman scattering from single crystal GaN films, both free-standing and attached to Al2O3 substrates, was performed over the temperature range from 78 to 800 K. These measurements reveal that the Raman phonon frequency decreases and the linewidth broadens with increasing temperature. This temperature dependence is well described by an empirical relationship which has proved to be effective for other semiconductors. The experiments also demonstrate that the strain from Al2O3 substrates compresses the epitaxial GaN in the c-axis direction.

The electron-beam-induced reduction of transition metal oxide surfaces to metallic lower oxides

Abstract The clean surfaces of several maximally valent transition metal oxides, namely TiO 2 , Nb 2 O 5 , V 2 O 5 and WO 3 , have been reduced to the corresponding binary oxide during extended observation inside a 400 keV high-resolution electron microscope. Selected-area electron diffraction patterns and optical transforms direct from the high-resolution lattice images were used to identify the oxide, while electron energy loss spectra from a sample of V 2 O 5 eliminated the possibility that the isostructural carbide had been formed. These reduced binary oxides have a defective rocksalt structure, and they are metallic in nature , which may thereby stifle further electron-stimulated desorption of oxygen.

Atomic imaging of oxide surfaces: I. General features and surface rearrangements

Abstract Surface profile images from a number of oxides have been obtained using high-resolution electron microscopy. Atomic-level details of the surface topography such as steps and terraces are clearly visible and, with the aid of image viewing and recording systems, cation redistributions on oxide surfaces can be followed in real time. Electron irradiation is shown to have different effects on the surfaces of different oxides. Transition metal oxides appear to desorb oxygen, effectively leading to “metallisation” of the surface, whereas holes are etched in alumina and nickel oxide, and surface rearrangements are the predominant effects seen in rare earth oxides.

New cesium titanate layer structures

A phase study of the Cs2Oue5f8TiO2 system in the composition range 75–100 mole% TiO2 and the temperature range 850–1200°C revealed the existence of two new cesium titanates, with compositions Cs2Ti5O11 and Cs2Ti6O13. The former compound undergoes a reversible hydration reaction below 200°C to form Cs2Ti5O11 · (1 + x)H2O, 0.5 < x < 1. The structures of the three phases have been determined. They are based on corrugated layers of edge-shared octahedra, with cesium ions (and H2O) packing between the layers. In Cs2Ti6O13, the layers are continuous in two dimensions, whereas in Cs2Ti5O11 and Cs2Ti5O11 · (1 + x)H2O, the layers are periodically stepped to give 5-octahedra wide, corner-linked ribbons.

The hollandite-related structure of Ba2Ti9O20

Abstract The crystal structure of Ba2Ti9O20 has been determined by comparison of experimental high-resolution electron micrographs with images simulated using structural models deduced from the micrographs in conjunction with crystallochemical principles. The structure consists of lamellae of hollandite-type structure alternating with BaTiO3-like units, which effectively immobilize the Ba ions. This material should be relatively more leach resistant to attack by aqueous sodium chloride solutions. The structure determination clarifies the observation that this material has unique properties as a microwave resonator, compared with other barium and alkali titanate structures.

Surface facetting and polarity of alumina

Abstract Atomic-resolution electron microscopic images of ruby show (0001) polar surface facets developing under electron irradiation. Attempts to understand the “dark-line” contrasts associated with the surface atom profiles, following comparison of the experimental images with computer modelling, leads to the realization that aluminium atom and oxygen atom surface terminations may be distinguished. Hence surface polarity, as well as details of surface step structure and mobility, may be determined. Positive surface polarity is preferred.

Electron irradiation effects in (Cs,Ba)-hollandites

High-resolution electron microscopy examination of mixed (Cs,Ba)-hollandites reveals evidence for significant short-range order in the distribution of cation vacancies in the incompletely occupied tunnels. The structure was observed to be quite unstable under 400-keV electron irradiation after only a few minutes study at a dose rate of ca. 10/sup 4/ electrons A/sup -2/ sec /sup -1/. Transformations to a microtwinned phase having interaxial angles of 80/sup 0//110/sup 0/ rather than 90/sup 0/, and finally to an amorphous substance, were directly observed and recorded using video techniques. It is evident that hollandites are structurally, chemically, and mechanically unstable under intense electron irradiation. The significance of these observations to the possible effects of self-irradiation in SYNROC preparations, where radioactive /sup 137/Cs/sup +1/ transmits to /sup 137/Ba/sup 2 +/ by electron decay, is discussed.

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

Abstract 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.

POINT, LINEAR AND EXTENDED DEFECT STRUCTURES IN NONSTOICHIOMETRIC RUTILE.

Abstract TiO2-x crystals (0 < x ≤ 0.0035) prepared at 1323 K and given controlled thermomechanica treatment were examined by high-resolution electron microscopy (HREM). Crystallographic shear planes (CSP) were not present in non-deformed specimens quenched from 1323 K but appeared, mostly as closely-spaced pairs, in slowly-cooled or deformed and reduced specimens. Lateral and longitudinal disorder in the fine structure of the CSP, which was strongly dependent upon cooling rate was observed. Platelet defects, approximately parallel to {100}, formed at about 400–600°C. Defects intermediate in size between point defects and CSP were also discovered. In situ observations revealed some details of the mechanisms of formation and dissolution of CSP and platelets. New interstitial cation structural models were derived using the above observations and electrostatic energy arguments. Linear defects, consisting of two face-shared pairs of octahedra containing trivalent cations are proposed. These have very much lowe...

The Nature and Extent of Disorder Within Rapidly Cooled TiO

A rapidly cooled sample of reduced rutile, composition TiO1.9985, has been examined by high resolution electron microscopy at 500 kV, and images of crystallographic shear planes (c. s. p.) and other defects have been obtained in three major projections, namely [¯111], [¯112] and [001], Close examination of high magnification images of individual c. s. p. along [111] indicated the presence of extensive disorder. Consideration of structural models for the disorder, particularly in relation to the electron-optical imaging conditions, indicated that its nature and extent might be determined directly from appropriate high resolution images. Indeed, analysis indicated that the defect configurations along the various c. s. p. could be sequenced, which enabled an overall statistical impression of the disorder to be obtained. The mean orientation of the crystallographic shear (c. s.) defects was very close to {132}, but {132}-, {143}- and {121}-type segments were observed to occur with approximately equal frequency, with smaller numbers of other types, including {110}. The longitudinal ordering of c. s. steps was thus primarily very short-range. Similarly, a wide variation in lateral (dis)order was indicated by the range of defect-image widths observed, and many c. s. p. occurred as pairs rather than as isolated defects. The extent of this disorder in the c. s. defects appears consistent with a strong influence of the cooling rate on the c. s. p. formation. The methodology developed for sequencing the c. s. defects and analysing the disorder should prove invaluable in obtaining a fuller understanding of reduction-deformation reaction mechanisms in reduced rutile and other systems.