Confirmation of an ANNNI-Like Model for Polytypism in SiC

The remarkable similarity between the observed SiC polytypes and the phases in the ANNNI model has suggested that the polytypes are equilibrium phases at high temperatures. Their origin lies in a competition between the nearest ( J 1 ) and next-nearest ( J 2 ) neighbour interactions between SiC double layers. We find, using norm-conserving pseudo-potentials, J 1 = +0.0050 eV and J 2 = -0.0034 eV per SiC pair. The sign of J 2 and the value of the ratio J 1 /| J 2 | are such as to lead to polytypes, thus broadly confirming the correctness of an ANNNI-like mechanism. The origins of J 1 and J 2 are discussed.