Study of relative energy of layer packing in polytypic crystals, using semiempirical methods

Abstract

An attempt is made to correlate some of the observed features in the layer stacking sequences of the polytypes of SiC, ZnS and CdI₂ based on energetics at the atomic and molecular level. Semiempirical potential functions have been used for the atomic interactions and the difference in energy of the cubic and hexagonal packing has been estimated for the different cases. The observed relative proportions of these two types of packings is shown to be governed by the above difference in energy. The results indicate that the occurrence of numbers such as 2, 3 in the Zhdanov symbol of SiC and larger numbers in the Zhdanov symbol of ZnS are a consequence of the slightly lower energy favouring the cubic packing. In the case of CdI₂ the occurrence of numbers 1 and 2 in the Zhdanov symbol is explained by the almost equal energy of packing of the sandwiched layers. The possibility of observing polytypism in the structure of diamond is pointed out. These results indicate that among various possible factors governing the phenomenon of polytypism, the energetics at the atomic and molecular level are important to be considered.