A. R. Lang

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 structure of coated diamonds

Abstract A typical coated diamond has been examined by x-ray diffraction and absorption topography. At the commencement of the coat an abrupt change to fibrous, radial growth occurs. This radial mode of growth gives rise to the general spherical habit of thickly coated diamonds. In the specimen studied the range of misorientation in the fibrous coat was 0·43° about the fibre axis and 0·25° about a normal to the fibre axis. Fibre diameters ranged up to some tens of microns. X-ray absorption contrast produced by the more opaque layers in the coat indicated maximum local impurity concentrations of 0·7% MgO or SiO2, or a few tenths of 1% of a heavier element such as Zr; but the concentration of Fe was less than 0·03%.

Linearly polarized luminescence from linear defects in natural and synthetic diamond

Abstract Photomicrographic studies of luminescence from cut and polished surfaces of natural and synthetic diamonds stimulated by electron bombardment (cathodo- luminescence) reveal sharply defined blue-emitting lines which, it is believed, correspond to dislocations. The blue emission is linearly polarized with electric vector always parallel to the line. The degree of polarization is greatest (90% or more) when the lines are straight and oriented in 〈110〉 directions and this orientation is frequently observed.

On giant screw dislocations in ZnS polytype crystals

Abstract Vapour-grown platelet crystals of ZnS containing long-period polytypes have been found by x-ray topography to contain a single screw dislocation with a very large Burgers vector parallel to the c axis. Individual polytype regions can have highly perfect lattices; the few dislocations observed in them in addition to the giant screw have Burgers vectors in the basal plane. A divergent-beam section topograph arrangement has been used to measure giant Burgers vectors to an accuracy of about 30%. Within this uncertainty, the Burgers vector magnitudes match the polytype repeat period in the five crystals upon which measurements have been made, the repeat periods ranging from 50 A to 106 A. The x-ray topographic studies provide good evidence in support of the periodic slip process for polytype formation.

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.

Cavitation as a Mechanism for the Synthesis of Natural Diamonds

LET us grant, in accordance with Galimovs proposals1, that cavitation can occur when flowing magma in a pipe encounters a constriction, and that in the ensuing collapse of a bubble very substantial transient dynamic pressures, of magnitude sufficient to be of thermodynamic importance for diamond synthesis, can be produced. In granting this, we overlook some quantitative details in his calculation, such as the apparent implication that the bubbles would contain gas at 10 or 20 kbar, and yet that their compression (by a factor of 64,000 in volume) can be calculated by ideal gas theory. He ignores the fundamental difference in rate control between martensitic conversion of crystals from one modification to another, which makes a product of the same chemical composition as the starting material, and other processes of crystal growth requiring a composition change. In the former class, to which production of diamond by the action of shock waves on graphite belongs, the limit on growth velocity is essentially the shock wave velocity. Thus, so far as that is concerned, quite a large diamond might be made within a few microseconds. Further characteristics of martensitic processes are, however, that the product takes the form of thin lenses, whereby the constraint by the matrix on shape change in the converting region is minimised and, second, that as a rule there is a multiplicity of orientations of the martensitic product in the parent crystal, so that a microcrystalline product results.

On the correspondence between cathodoluminescence images and X-ray diffraction contrast images of individual dislocations in diamond

Abstract A one-to-one correspondence has been demonstrated between dislocation lines outcropping at or lying not deeper than circa 20 μm below the specimen surface, as mapped by X-ray topography, and lines emitting a characteristic linearly polarized blue cathodoluminescence which can be seen in regions unusually sparsely populated by other defects giving rise to visible cathodoluminescence upon excitation by electrons in the energy range 10 to 50 kV.

A study of repeated twinning, lattice imperfections and impurity distribution in amethyst

Abstract A basal-plane slice of a crystal possessing the amethyst structure of laminated Brazilian twinning in the major rhombohedron growth sectors has been studied by x-ray diffraction topography and absorption topography. The twin boundaries lie in the [0001] zone. Parallel with them lie strongly diffracting lamellae of imperfect crystal containing a high density of decorated dislocations and the highest iron impurity concentration in the slice. The decorated dislocations generally make fairly small angles with the basal plane and appear to pass through the lamellae. X-ray absorption shows that the major rhombohedron growth sectors contain on average 0·04 weight per cent more iron than the minor rhombohedron growth sectors in the specimen. The imperfect lamellae contain about 0·025% more iron than the regions between them.

On the macroscopic distribution of dislocations in single crystals of high-purity recrystallized aluminium

Abstract Single crystals about 1 cm2 in area and 1 mm thick prepared by annealing both lightly and heavily deformed zone-refined aluminium have been examined x-ray topographically. From stereopairs of various Bragg reflections the orientation and Burgers vectors of dislocations could be determined. Repetitive examination showed dislocation movements after slight deformation and also after long storage. Helices and sets of coaxial prismatic loops are seen with diameters ranging from a minimum observable value of about 1 μm to over 60 μm, and they may extend more than 1 mm along 〈110〉 directions. These features result from absorption of vacancies by long screw dislocations which, it is concluded, constituted a major part of the dislocation population grown-in during recrystallization. Dislocation densities of only a few tens per cm2 are found within 200 to 300 μm of the specimen surfaces and evidence is presented that such low densities arise from loss of dislocations at the surfaces.

Cathodoluminescence evidence of dislocation interactions in diamond

Abstract Dislocation cathodoluminescence exhibiting certain peculiarities in anisotropy of polarization localized in sheets parallel to octahedral planes in a mosaic, semiconducting natural diamond is interpreted as a macroscopic manifestation of microscopic interactions between glide dislocations on these octahedral slip planes and the dislocations of the mosaic, whereby passage of fresh glide dislocations through low-angle boundaries and forest dislocations of the mosaic has produced jogs which have persisted since the deformation.