Z. H. Kalman

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...

Polar Properties of ZnS Crystals and the Anomalous Photovoltaic Effect

ZnS platelets grown from the vapor phase, bounded by (211), (011), and (111) faces, were used. The crystals showed birefringence bands perpendicular to the [111] axis. X‐ray rotation, oscillation, and Laue photographs showed that the bands are due to changes in crystal structure (cubic, hexagonal, and polytypes) and one‐dimensional stacking disorder. The polarity of the common [111] axis of the various bands was determined by several techniques. On the (211) faces, heating with H2O2 caused the appearance of triangular etch pits, aligned with bases parallel to the [011] direction and apexes pointing all in the same sense. The (111) face towards which the triangles pointed was attacked faster by the etchant than was the opposite one. The same (111) face became negatively charged if a (211) face was uniformly illuminated by 343 mμ radiation. Furthermore, the same face turned out to be a zinc plane, as revealed by comparing the intensity of two (111) reflections, using wavelengths at both sides of the Zn...

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.

Growth and crystal structures of solid xenon and krypton

Abstract A study is reported of the growth and crystal structures of vapor-phase grown polycrystalline samples of xenon and krypton. X-ray diffractometry and powder photography establish that two structures (the well known fcc and the recently discovered hcp phases) co-exist over a large temperature range and under various conditions of growth. Microscopic investigation reveals surface features such as parallel striations and etch pits. The results indicate that fcc is the stable phase of these solids, whereas hcp is metastable. A possible model of growth is described.

Single crystal growth of Rb2MnCl4 and Rb3Mn2Cl7

Abstract Crystal growth by the vertical Bridgman technique of the incongruently melting Rb 2 MnCl 4 and Rb 3 Mn 2 Cl 7 is reported. Large single crystals of optical quality could be grown using non-stoichiometric melts. The crystals are characterized by chemical analysis, density measurements, X-ray powder patterns and cleavage planes.

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.

Laue photography of fine crystalline particles

An experimental method is described for the determination of crystallite dimensions in polycrystalline materials. It is based on studying the geometry of streaks that appear in powder photographs taken with white X-ray radiation. A detailed analysis is presented that includes a differentiation between these streaks and somewhat similar streaks due to asterism. Results by synchrotron radiation from powder samples of various materials are discussed and compared with the crystallite dimensions obtained by other methods. It is demonstrated that the method is largely complementary to measurements by electron microscopy and that it is particularly useful for the determination of the thickness distribution of thin (0.005–0.5 μm) platelets, oriented parallel to the substrate.

On the inapplicability of the layer-transposition mechanism to polytype formation in ZnS

The frequency distribution of the relative lengths of cubic stacking sequences in vapour-phase-grown ZnS polytypes is presented. It is shown that this distribution conflicts with the correlation between atomic structures and stacking-fault energy expected on the assumption of a layer-transposition mechanism for polytype formation.