On structure and twist in ZnS crystal whiskers

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Abstract
ZnS crystal whiskers grown from the vapour at about 1450 K, with typical lengths of about 20 mm and diameters about 100 μm, have been studied by synchrotron X-ray topography. Crystallographic structure and texture have been compared with morphological and optical characteristics point-by-point along whisker axes. Using continuous radiation and cylindrical camera geometry, entire diffraction patterns from zones of type have been recorded, providing information on lattice symmetry and polytype order as well as on local lattice perfection in the whisker. The X-ray topographs spanned the three main types of structure in vapour-grown ZnS whiskers, namely an initial region of usually faulted 2H structure, a following region (of ‘striated’ appearance, optically) consisting of a stack of lamellae (average thickness a few μm) of polytypic structures with various c-axis repeat periods, and a third, following region containing long-period polytypes, all of the same order, in which individual structures can occupy lengths up to millimetre dimensions along the c axis and behave as perfect crystals except for the presence of a giant screw dislocation along the c axis. The third region exhibits the Eshelby twist expected from an axial screw dislocation with Burgers vector of magnitude equal to the axial length of the elementary stacking sequence of ZnS layers in the polytype structure, an equality accurately confirmed in two specimens by counter-diffractometer measurements with Cu Kx 1 radiation. The region of thin lamellae can contain polytypes of high order equal or close to that found in the more perfect third region, but it shows inconstancy of polytype order along the c axis: from symmetry properties of the diffraction images on the X-ray topograph it is seen that all polytypes are of even order, changes of polytype order taking place by ± 2m unit layers of ZnS structure (m being a small integer, frequently unity). In the four specimens studied, this lamellar region does not exhibit the expected Eshelby twist.

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