Abstract

Various mechanisms for polytypic growth in zinc sulphide crystals have been discussed. It has been pointed out that the polytypic growth in this compound is the result of the ordering of f.c.c. microtwins along the <111> directions. A comparison between the structures actually found and those expected from consideration of the periodically repeated f.c.c. type microtwins shows a good agreement. The periodic occurrence of f.c.c. type microtwins has been shown to result from 2H structures containing growth faults only, when faults due to slip at alternate layers and axial screw dislocations operate simultaneously. This mechanism is assumed to operate only at a stage when crystals are growing, and it can successfully predict almost all structures which are actually found in ZnS polytypes. Only a few polytypes (three, 9R, 12R, and 21R, out of more than a hundred) with structures not fitting the above mechanism have been attributed to the insertion of periodic stacking faults in some other polytypes (which are generated in accordance with the proposed mechanism) at a later stage of the crystal growth.