Interferometric and X-ray investigation of polytypism in cadmium iodide crystals in relation to crystal growth

Abstract

A combined optical and X-ray diffraction study has been undertaken of the phenomenon of polytypism in cadmium iodide crystals, which grow by a dislocation mechanism. 88 CdI2 crystals have been investigated to study Frank's ideas of a 'buckle followed by slip' mechanism in the creation of dislocations and subsequently of polytypes. A correlation between Che spiral step height and X-ray unit cell is found for a few 2H and 4H structure types. However, for polytypes with larger unit cells no such correlation was observed. This is at variance with previous observations and with Frank's theory and cannot yet be fully explained. Some X-ray diffraction photographs show 'diffuse background' and 'smearing of diffraction spots', with spots elongated along streaks, similar to Jagodzinski's observations on silicon carbide crystals. 13 (possibly 18) new polytypes of CdI 2 are described. In addition, several crystals resembling 2H and 4H types, but having either intensity differences or conspicuous absences of certain reflections, have been found.

Extended structural defects in ?-Hgl2 single crystals

“…14a grown-in forest dislocations, originating from the seed and escaping the screen, are revealed as close-spaced dark dots The very long lines [131] and the large width [132] of the glide double sheets of dislocations indicate that the A grains underwent a low rate of strain; they were deformed corresponding to the transition region between deformation stages I and II [129][130][131][132][133][134]. Deformations by rotational slip giving rise to basal plane twist boundaries, already observed in crystals having a layered structure such as CdI 2 or graphite [135][136][137][138], are also common in 0~-HgI 2 crystals. The density of grown-in forest dislocations, emerging on the {1 0 1} faces, in solution-grown crystals using the screened seeding technique, is very low, of the order of 50-100 cm -z.…”

“…39d. This behaviour might be explained by assuming the formation of growth domain boundaries in the crystals during their growth, since antiphase boundaries introduce a random increase of the lattice constant a while stacking faults (stacking 1) should not produce an important increase of the lattice constant c. Alternatively, the observed streaking may be explained using basal plane screw boundaries [172] or more generally by the glide polygonization that mostly affects the a lattice constant. The arcing of the spots 100-104 and 257, 258, 2 5 9, 2 0 6, 2 0 8, seen in Figs 39b and c, respectively, and the splitting of the spots 3 12 and 4 12, seen in Fig.…”

Studies on a new polytype of cadmium iodide 28Hc

“…However, for an unequivocal verifi. cation of Frank's dislocation theory of polytypism in the case of cadmium iodide, Trigunayat & Verma (1962) tried to find a correlation between the spiral step-height and the height of the unit cells by performing both the X-ray diffraction and the interferometric studies upon the same crystals. They found that no such correlation exists.…”

Polytypism and Stacking Faults in Crystals with Layer Structure

“…Such a correlation was observed by Verma for 7 SiC polytypes [146]. Making use of optical data of Forty [147] on step height measurement, Mitchell [43,148] sought to establish a similar correlation for a large number of solution-grown CdI z polytypes examined by him through X-ray diffraction methods, which, subsequently, has been negated by Trigunayat and Verma [49], and later by Chadha and Trigunayat [149] in vapour-grown crystals, who performed the two kinds of measurement on the same crystal. Such a verification often demands a high degree of accuracy in step height measurement, which is permitted in CdI2 crystals because they often present a multitude of internal interference fringes contouring the spiral steps, e.g.…”