S. Mardix

The mechanism of polytype formation in vapour-phase grown ZnS crystals

Abstract Morphological, x-ray crystallographic and x-ray topographic studies on many specimens grown in this laboratory lead to a consistent description of polytypism in vapour-phase grown ZnS. Crystal growth begins with the emerging of hexagonal 2H needles from the polycrystalline substrate. The needles thicken during the growth and often expand into platelets. The growth of the needles takes place around a screw dislocation with a Burgers vector 2nc D, where C 0 is the interlayer distance along the c axis. Consequently the (00.1) layers form a set of interleaved helical surfaces. Usually n= 1, but occasionally n> 1 and in this case subsequent formation of a poly-type is possible. This takes place by means of stacking faults which are introduced while the specimens are cooling down in the growth tube. Guided by the helical topology the stacking faults expand onto many equidistant (00.1) layers and thus transform the structure. It is demonstrated that all known features of ZnS polytypes in vapour-phase gr...

Periodic slip processes and the formation of polytypes in zinc sulphide crystals

Inherent deformations of ZnS polytype regions are determined by measuring angles between linear markings. The deformations are correlated to the crystallographic structures of the polytypes as well. The relations clearly indicate that polytype formation occurred by a periodic slip process after crystal growth had been completed. Further observations substantiating this model are quoted.

The family 24L of ZnS polytypes

constants used by Giglio show that configuration B is more stable than configuration A, in accordance with the experimental evidence. Although the result obtained here is sensitive to the choice of the various parameters, in particular those of repulsive energy, it is certain that the interactions between non-bonded atoms are important in the determination of the geometrical relationship between adjacent TCNQ molecules (Goldstein, Serf & Trueblood, 1968). As mentioned above, TCNQ molecules are stacked face-to-face to form columns of monadic units of TCNQ. The same monadic unit has also been found in crystals of N-methylphenazinium(TCNQ). This fact seems to be closely related to the fact that the cations are planar (aromatic in both cases). It is noteworthy that both salts are among the best electrically conductive organic compounds hitherto reported. The calculations were performed on the HITAC 5020E computer at the Computer Centre of this University with a universal crystallographic computation program system, UNICS, and on the FACOM 270-30 computer at this Institute.

The identification of high-order polytypes

A practical and fast method for the determination of the layer sequence of high-order polytypes is described. Experimentally, the method involves only the determination of the relative order of reflexion intensities and does not employ actual intensity measurements. Auxiliary information, such as the percentage of hexagonality and the cyclicity of the polytype involved, considerably shortens the computer time needed. The method was used to identify a large number of ZnS polytypes and it is applicable to other polytypic material as well.

High‐order ZnS polytypes and their identification

A list of ZnS polytypes having thirty or more layers in their elementary stacking sequences is presented, including seven new polytypes: 90R (18 2 7 3)3; 90R (8 3 5 5 3 6)3; 96R (17 4 6 5)3; 102R (31 3)3; 34L (7 5 3 5 5 5 2 2); 120R (29 3 2 2 2 2)3 and 120R (13 3 3 5 11 5)3. Polytype identification procedures are discussed from the point of view of reliability. It is demonstrated that even for such polytypes unambiguous identification is possible without exact intensity measurements. For these polytypes, the Burgers vector of the generating [00.1] screw dislocation has a length of 90/~ or more.