Changes of the geometry and band structure of SiC along the orthorhombic high-pressure transition path between the zinc-blende and rocksalt structures

Using the first principles pseudopotential plane-wave method and the full-potential linearized muffin-tin orbital method, we study how the geometry and the electronic structures change along the orthorhombic transition paths of zinc blende (ZB) to rocksalt (RS) under high pressure. Two different paths, called the fixed strain (force-free) path and the fixed position (hydrostatic stress) path, pass both through the same transition state at any pressure. The actual transition point, however, depends on pressure. The force free path shows a turning point where the atoms jump to the RS positions. A stronger response to changes in either the intersublattice displacement or the strain is observed near the transition state than near the end phases. A phenomenological model helps to reveal that the transition state (TS) is the result of the long-range periodic dependence of the energy on the order parameter whereas the turning point is the result of the local dependence around RS. The band structures show that the TS is metallic although both ZB and RS are semiconductors. This explains the softening of the optical phonons under large strains and why the energy barrier for the relative movements of the sublattices is very low when the strains are kept at the TS.