On Polytypism and Internal Rotation

The ordered stacking of eclipsed (E) and staggered (S) layers in SiC or ZnS crystals leads to formation of polytype lattices. The short-range forces acting between layers are considered to be analogous to those determining the barriers to internal rotation in molecules. This is applied to an understanding of eclipsing in covalent lattices (SiC, diamond) and in the more ionic lattices such as ZnS. The Madelung energies are calculated for the prevalent polytype structures, and there is a small effect favoring a cooperative formation of the completely eclipsed lattice, supporting Schneer's proposal that the zincblende–wurtzite transition is second-order. Configurational and vibrational entropy effects are not found to contribute to the stabilization of polytype lattices. The progressive distortions in these lattices with increasing numbers of E layers are attributed to “internal-rotation” forces and may be the explanation of polytype formation. A simple model, successfully applied to ethane, is used to calculate possible distortions in the recently prepared hexagonal diamond lattice. An undistorted lattice, not significantly more stable than the cubic structure, was obtained. Electron–nuclear interaction terms dominate over nuclear–nuclear terms, just the opposite of ethane.