Microscopic description of the mechanism of transition between the 2$H$ and 4$H$ polytypes of silicon carbide

The mechanism of displacement of one close-packed SiC layer from one minimum position to another on the example of SiC polytype transition $2H\to4H$ has been studied by ab initio methods. It has been shown that the intermediate state with monoclinic symmetry $Cm$ greatly facilitates this displacement breaking it into two stages. Initially, the Si atom chiefly moves, only then–mainly the C atom. In this case, the Si–C bond is significantly tilted in comparison with the initial position, which allows the reducing of the compression of the SiC bonds in the (11$\bar2$0) plane. Two transition states of this process, which also possess the $Cm$ symmetry, have been computed. It has been found that the height of the activation barrier of the process of moving the close-packed layer of SiC from one position to another is equal to 1.8 eV. The energy profile of this movement has been calculated.