Moreton Moore

On the internal structure of natural diamonds of cubic habit

Abstract Three natural cubes, each of edge length about 1·8 mm. were examined by x-ray topography. Two possessed very similar internal structures, describable in terms of space-filling by fibrous or columnar single-crystal growth in directions, with branching and equal growth velocities in these symmetrically equivalent directions. On a microscopic scale this structure appears similar to that of diamond coat: the cubic habit results from the branching mode of growth when the final crystal size is large compared with the dimensions of its core or nucleus. The internal structure of the third cube was entirely different. The record of growth horizons and dislocation trajectories showed that during most stages of its growth it had developed with a combination of forms: normal {111} faces plus hummocky, non-crystallographic surfaces of mean orientation {100}. The latter had become dominant in the outer parts of the crystal but the final overall cubic shape had been accentuated by solution following growth.

On the origin of the rounded dodecahedral habit of natural diamond

Abstract The morphology of natural rounded rhombic dodecahedral diamonds has been studied optically and by scanning electron microscopy, and their internal structures have been investigated by X-ray topographic techniques. That this rounded habit arises from dissolution of a sharp-edged octahedral growth form is indicated both by theoretical considerations and experimental evidence. The hypothesis that these diamonds grew throughout their history as rounded rhombic dodecahedra is directly refuted by the X-ray topographic studies. The information on dislocation distribution revealed by X-ray topographs also provides strong evidence against a current variant of the growth hypothesis, that the rounded rhombic dodecahedral habit developed upon an octahedral core. The manner by which dissolution of surfaces of rounded dodecahedral diamonds proceeds can be described on an atomic scale by motion of kinks along stable monomolecular steps parallel to 〈110〉 directions. It is shown that this process leads to the development of edges parallel to the minor diagonals of the rhombic surfaces, in accord with observation.

Imaging diamond with x-rays

The various techniques for imaging diamonds with x-rays are discussed: x-radiography, x-ray phase-contrast imaging, x-ray topography, x-ray reciprocal-space mapping, x-ray microscopy; together with the characterization of the crystal defects which these techniques reveal.

Synchrotron X-ray topography

Abstract The various techniques of X-ray diffraction topography image imperfections in single-crystals by Bragg reflexion, with a spatial resolution of approximately one micrometre. Defects can be studied in relation to crystal growth and physical properties. X-ray interference effects can also be explored in perfect, and nearly perfect, crystals. Synchrotron radiation has given X-ray topography additional powers, including the rapid non-destructive assessment of crystal perfection in materials of industrial importance.

Twinning in Natural Diamond. II. Interpenetrant Cubes

Single crystals of natural diamond which exhibit an approximately cubic morphology are known to have grown either (a) by nonfaceted cuboid growth or (b) by fibrous growth branching in a multiplicity of 〈111〉 columns, to give a mean surface orientation of {100}. Interpenetrant twins of diamond are uncommon; but when they occur they are usually coloured and appear (by X-ray topography) to have been formed by fibrous growth upon a twin origin, with the 14 growth sectors adequately filled by fibres, without the necessity of branching.

On the dodecahedral growth of coated diamonds

Abstract A study has been made of coated diamonds which exhibit rhombic dodecahedral facets in addition to the more usual octahedral and cubic faces of such crystals. X-ray topography showed that the dodecahedral facets had developed by branched fibrous (columnar) growth along octahedral directions: schematically, [111]+[11 1 ] → (110). Scanning electron microscopy has shown that the dodecahedral facets were linearly corrugated on a fine scale, with trigons present on the sloping sides of the corrugations. This observation complements the X-ray topography, demonstrating that the dodecahedral facets are in fact made up of fine-scale {111} surfaces. The rhombic dodecahedral is thus shown here not to be a true growth form for coated diamonds.

White-beam X-ray topography

After radiography, white-beam X-ray topography (XRT) is the simplest X-ray imaging technique for crystals. An X-ray topograph is formed by a Bragg reflexion and is in effect a high-spatial-resolution Laue ‘spot’. Synchrotron radiation has given XRT additional powers, with its broad continuous spectrum, small beam divergence, high intensity, strong polarization and regular pulsed time structure. Each Laue image, however, may consist of the superposition of several harmonic reflexions. Background scattering should be attenuated to improve signal-to-noise ratio; and Laue spots should not overlap. This may impose a minimum distance between specimen and detector, possibly at the expense of spatial resolution. Account may need to be taken of absorption of the X-rays, both in the crystal and in air. The configuration of the experiment must be chosen carefully to minimize geometrical image distortions. The detector may be an image plate or a charge-coupled device; but for the best resolution (1 or 2 µm), a photographic plate is used to image individual dislocations. Whole specimens may be illuminated with synchrotron X-rays, or regions may be selected by slits. In the former case, crystals may be checked for distortions or for twinning. In the latter, an image may be made of a slice precisely parallel to a chosen crystallographic plane, as the crystal selects the appropriate ‘Bragg wavelength’ from the continuous spectrum: (a technique only available to characteristic radiation in special cases). Crystalline defects and strains associated with crystal growth, plastic deformation, phase transitions, polytypes and magnetic domains may be studied non-destructively by XRT.

An X-ray topographic study of impurity platelet distribution and size in an inhomogeneous diamond

Abstract A natural diamond was shown by x-ray topography to consist of an included diamond exhibiting strong evidence of slip on {111} planes which was surrounded by a parallel overgrowth in which only local and weak signs of slip were seen. A topographic study of intensities of the anomalous diffuse ‘spike’ reflections demonstrated that the inclusion was relatively poor in nitrogen impurity platelet precipitates compared with the overgrowth. It way shown how platelet diameters could be measured from the sharpness of ‘spike’ topograph images, and an application of the technique indicated that platelet diameters in the outermost parts of the diamond were in the range 150 to 200 A. Sequences of depth, temperature and stress conditions which could have been responsible for the observed structure of the diamond are discussed.

Positron annihilation spectroscopy applied to porous silicon films

A monoenergetic slow positron beam has been used for the first time to profile porous silicon films. High values of the Doppler‐broadened line shape parameters are observed, which correspond to positron annihilation within the porous layers and these are attributed to the decay of para‐positronium. After allowing for the reduced density of the porous film, fitted values of thickness were deduced which were in reasonable agreement with values obtained from ellipsometry measurements. Low values of the Doppler parameters observed for the two samples with the thinnest films are attributed to oxide residing at the interface of the porous and bulk silicon regions. Etching the samples in a solution of 48% hydrogen fluoride reduced the porous film thickness significantly, suggesting that a considerable amount of the film consists of SiO2. A four‐component convolution analysis routine is used to analyze the individual 511 keV annihilation peaks, the result of which confirms the formation of positronium within the ...

Observations of Borrmann-Lehmann Interference Patterns with Synchrotron Radiation

The intrabranch interference effects expected under Laue-Bragg diffraction conditions [Saka, Katagawa & Kato (1972). Acta Cryst. A28, 102-113, 113-120] and first studied experimentally under high-absorption conditions by Borrmann & Lehmann [Crystallography and Crystal Perfection (1963), edited by