The formation of double-polytype regions in ZnS crystals

A 3 mm long double-polytype region was found in a ZnS needle. The region consists of a fine mixture of 47% of each of the polytypes 20L(5753) and 60R(5537)₃ and about 6% of the polytype 60R(511553)₃. The latter polytype is assumed to be the parent polytype transforming into the two others. It is the first time that traces of the parent polytype have been found in a double-polytype region. The formation mechanism of double-polytype regions is explained as a two-stage transformation process: expansion of grown-in stacking faults taking place at temperatures just below the hexagonal-to-cubic transition temperature, followed by a sudden formation and expansion of stacking faults at a lower temperature. It is suggested that a similar mechanism operates in the formation of single polytype regions and disordered ones.