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An earlier report [Acta Cryst. (1977), A33, 681-684] by a joint IUCr-IMA committee on the nomenclature of polytypism has been revised and extended. Two kinds of symbolism are recommended for use with either simple or complicated polytypic structures. The first consists of 'indicative symbols' in a modified Gard notation, the second of 'descriptive symbols' based on earlier proposals by Dornberger-Schiff, I)urovi~ and Zvyagin. The polytypism of ZnS and SrGeO3 provides examples for the use of descriptive symbolism.

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 K S , 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 a t 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 2R ZnS crystals. The point intensity distribution along the 1O.L reciprocal lattice row of a slightly faulted 2H crystal is recorded on a four-circle single-crystal diffractometer in steps of AL = 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 werdenRGntgenbeugungsuntersuchungen an den 2H-3C-Festkorper-Strukturubergiingen in ZnS durchgefiihrt. 2H-ZnS (Wurtzit)-Einkristalle, aus der Dampfphase oberhalb 1100 "C in H,S-Atmosphiire gezfichtet, werden im Vakuum bei verschiedenen Temperaturen im Bereich von 300 bis 650 "C getempert. Es wird gefunden, daB der ffbergang mit dem statistischen Einsetzen von Stapelfehlern ubereinstimmt, wie aus der Vermehrung der diffusen Streifen liings der Reihen parallel zu c* im reziproken Gitter gefunden wird. Fiir die 3C-Struktur charakteristische Beugungspunkte erscheinen auf den Streifen in einem spiiteren Zustand, und das Endprodukt ist invariabel eine fehlgeordnete verzwillingte 3C-Struktur. Die Transformationsrate hiingt sowohl von der Temperungstemperatur ale auch von der Perfektion des Ausgangskristalls ab. In der iiuBeren Form des Kristalls wird keine h d e r u n g beobachtet. Um die Art der bei dem ffbergang beteiligten Stapelfehler zu bestimmen, wird die Verbreitemg der Rontgenbeugungsmaxima infolge der Temperung der 2H-ZnS-Kristalle untersucht. Die Punktintensitiitsverteilnng in Richtung der 1O.L-Reihe des reziproken Gitters einea schwach fehlerhaften 2H-Kristalls wird mit einem Vier-Kreis-Einkristall-Dif~aktometer in Stufen von AL = 0,Ol aufgezeichnet. Es wird gefunden, daB die Halbwertsbreiten der geraden und ungeraden Reflexionen gleich sind, was darauf hinweist, daB die wiihrend der Temperung eingebrachten Stapelfehler uberwiegend Deformatio...

The nature of random stacking faults in a heavily disordered single crystal of 2H SiC has been investigated by studying the broadening of X-ray diffraction maxima. The intensity distribution along the 10.l reciprocal lattice row was recorded on a four-circle, computer-controlled single crystal diffractometer. The 10.l reflections with l even were found to be considerably broadened showing that the stacking faults present are predominantly intrinsic faults. Exact expressions for the diffracted intensity and the observable diffraction effects were obtained. The results obtained are compared with those obtained for 2H ZnS crystals.

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