Dislocations are common defects in solids, yet all crystals begin as dislocation-free nuclei. The mechanisms by which dislocations form during early growth are poorly understood. When nanocrystalline materials grow by oriented attachment at crystallographically specific surfaces and there is a small misorientation at the interface, dislocations result. Spiral growth at two or more closely spaced screw dislocations provides a mechanism for generating complex polytypic and polymorphic structures. These results are of fundamental importance to understanding crystal growth.

http://science.sciencemag.org/content/sci/281/5379/969.full.pdf

Imperfect Oriented Attachment: Dislocation Generation in Defect-Free Nanocrystals

THE properties and potential applications of carbon nanotubes filled with other materials have aroused much speculation1–5. Strategies for filling nanotubes include in situ growth in an arc reactor using metal/carbon composites2,5 and the capillarity-driven filling of open nanotubes using liquid reagents3,4. Here we report an alternative approach to the synthesis of nanoscale structures based on nanotubes, in which the tubes are converted to carbide rods by reaction with volatile oxide and/or halide species. In this way we have been able to prepare solid carbide nanoscale rods of TiC, NbC, Fe3C, SiC and BCx in high yield with typical diameters of between 2 and 30 nm and lengths of up to 20 urn. Preliminary studies show that these rods share the properties of the bulk materials (such as magnetism and superconductivity), suggesting that they might allow the investigation of the effects of confinement and reduced dimensionality on such solid-state properties. These carbide nanorods might also find technological applications in nanostructured composite materials.

Synthesis and characterization of carbide nanorods

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jps.2600580802

Pharmaceutical Applications of Polymorphism

Solid-State Structures of Hydrogen-Bonded Tapes Based on Cyclic Secondary Diamides

It has been recognized that Raman scattering spectroscopy is a powerful tool to characterize SiC crystals non-destructively. We review recent significant developments in the use of Raman scattering to study structural and electronic properties of SiC crystals. The areas to be discussed in the first part include polytype identification, evaluation of stacking disorder and ion-implantation damages, and stress evaluation. The Raman scattering by electronic transitions is discussed in the second part of this article. We concentrate on the plasmon LO-phonon coupled modes whose spectral profiles are used to evaluate the carrier concentration and mobility. Anisotropic electronic properties of α-SiC and characteristics of heavily doped crystals are discussed. Semiconductor-to-metal transition and Fano interference effect are also treated.

Raman Investigation of SiC Polytypes

Polymorphism and Polytypism in Crystals

Random, Non-Random and Periodic Faulting in Crystals

Mitchell has deduced the polytypic structures that would arise from theoretical screw dislocations in the 6H, 4H and 15R phases of silicon carbide, by the spiral-growth mechanism of Frank, and compared these with the structures actually observed. In doing so, he assumed that at the instant of the creation of a screw dislocation, the initial platelet consists of a completed number of unit cells and that the exposed ledge, irrespective of its own structure, can wind over itself in a close-packed manner, thereby repeating its own zig-zag sequence indefinitely during the subsequent spiral growth. It is shown that both these assumptions are not justified and that the use of Zhdanov symbols alone leads to incorrect results. The possible polytypic structures that can be derived from theoretical screw dislocations of different Burgers vectors are rededuced with corrected assumptions, by considering the different possible structures of the exposed ledge in terms of ABC layers and investigating the stacking of these layers during the subsequent spiral growth of the ledge. The results so obtained differ markedly from those of Mitchell. In particular a much larger number of structure series are shown to be actually possible. The results are compared with the polytypic structures reported so far.

On the deduction of silicon-carbide polytypes from screw dislocations

An X-ray diffraction study is made of the 2H 3C solid-state structural transformation in ZnS. Single crystals of 2H ZnS (wurtzite), grown from the vapour phase above 1100 °C in the presence of H2S2, are annealed in vacuum at different temperatures ranging from 300 to 650 °C. The transformation is found to commence with a statistical insertion of stacking faults as revealed by the intensification of the diffuse streaks along reciprocal lattice rows parallel to c*. Diffraction spots characteristic of the 3C structure appear on the streaks at a later stage and the end product is invariably a disordered, twinned 3C structure. The rate of transformation is found to depend on the annealing temperature as well as the perfection of the initial crystal. No change in the external shape of the crystals is observed. To determine the nature of stacking faults involved in the transformation the broadening of the X-ray diffraction maxima are investigated produced by annealing 2H ZnS crystals. The point intensity distribution along the 10.L reciprocal lattice row of a slightly faulted 2H crystal is recorded on a four-circle single-crystal diffractometer in steps of ΔL = 0.01. It is found that the half widths of the L even and L odd reflections are equal, indicating that the stacking faults introduced during annealing are predominantly deformation faults. The observed and calculated intensity profiles of different individual reflections are found to be in good agreement. A slight discrepancy observed is attributed to the non-random insertion of stacking faults during transformation.



Es werden Bontgenbeugungsuntersuchungen an den 2H-3C-Festkorper-Strukturubergangen in ZnS durchgefuhrt. 2H-ZnS (Wurtzit)-Einkristalle, aus der Dampfphase oberhalb 1100 °C in H2S- Atmosphare gezuchtet, werden im Vakuum bei verschiedenen Temperaturen im Bereich von 300 bis 650 °C getempert. Es wird gefunden, das der ubergang mit dem statistischen Einsetzen von Stapelfehlern ubereinstimmt, wie aus der Vermehrung der diffusen Streifen langs der Reihen parallel zu c* im reziproken Gitter gefunden wird. Fur die 3C-Struktur charakteristische Beugungs punkte erscheinen auf den Streuen in einem spateren Zustand, und das Endprodukt ist invariabel eine fehlgeordnete verzwillingte 3C-Struktur. Die Transformationsrate hangt sowohl von der Temperungstemperatur als auch von der Perfektion des Ausgangskristalls ab. In der auseren Form des Kristalls wird keine anderung beobachtet. Um die Art der bei dem ubergang beteiligten Stapelfehler zu bestimmen, wird die Verbreiterung der Rontgenbeugungsmaxima infolge der Temperung der 2H-ZnS-Kristalle untersucht. Die Punktintensitatsverteilung in Richtung der 10.L-Reihe des reziproken Gitters eines schwach fehlerhaften 2H-Kristalls wird mit einem Vier- Kreis-Einkristall-Diffraktometer in Stufen von ΔL = 0.01 aufgezeichnet. Es wird gefunden, das die Halbwertsbreiten der geraden und ungeraden Reflexionen gleich sind, was darauf hinweist, das die wahrend der Temperung eingebrachten Stapelfehler uberwiegend Deformationsstapelfehler sind. Die beobachteten und berechneten Intensitatsprofile von verschiedenen individuellen Reflexionen befinden sich in guter ubereinstimmung. Eine leichte Diskrepanz, die beobachtet wird, wird der nichtstatistischen Einfuhrung der Stapelfehler wahrend der Transformation zugeordnet.

X‐ray diffraction study of the 2H to 3C solid state transformation in vapour grown single crystals of ZnS

The 2H or AB. . . close-packed structure may be transformed to 6H or ABCACB. . . structure if layer displacement faults occur preferentially on every third close-packed layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H → 6H structural transformation by such a layer displacement mechanism is developed. It is shown that it is necessary to consider that the faults do not occur entirely at random but prefer to occur at three-layer separations from each other in order to statistically create a 6H structure. The diffraction theory, as developed by earlier workers for 2H crystals containing a completely random distribution of stacking faults, cannot therefore be applied to the present case. An exact expression for the diffracted intensity from crystals undergoing the 2H → 6H transformation has been obtained and the different observable diffraction effects (like change in integrated intensity, peak shift, integral breadth, peak asymmetry) have been predicted.

X-ray diffraction from one-dimensionally disordered 2H crystals undergoing solid state transformation to the 6H structure. I. The layer displacement mechanism

P. Krishna

Papers

Polymorphism and Polytypism in Crystals

Random, Non-Random and Periodic Faulting in Crystals

On the deduction of silicon-carbide polytypes from screw dislocations

X‐ray diffraction study of the 2H to 3C solid state transformation in vapour grown single crystals of ZnS

X-ray diffraction from one-dimensionally disordered 2H crystals undergoing solid state transformation to the 6H structure. I. The layer displacement mechanism