Pathways of transitions between polytypes in silicon carbide

In this work, two main polytypic transformations in silicon carbide, namely, $2H\to 6H$ and $3C\to 6H$, were studied using ab initio methods. It is shown that intermediate states with trigonal symmetry $P3m1$ and monoclinic symmetry $Cm$ greatly simplify the movement of close-packed layers during such transitions, breaking them up into separate stages. It was found that the two polytypic transformations occur quite differently. During the $2H\to 6H$ transition, the displaceable bonds are noticeably tilted compared to the initial position, which makes it possible to reduce the compression of SiC bonds in the (11$\bar2$0) plane. The $3C\to 6H$ transition takes place through the formation of auxiliary Si–Si and C–C bonds that live for a short time and help the tightly packed layers exchange places. As a result, the activation barrier for the conversion of $2H\to 6H$ (1.7 eV/atom) is significantly less than the activation barrier for the conversion of $3C\to 6H$ (3.6 eV/atom), which means that the second transition must occur at temperatures of 750–800$^\circ$ higher than the first one. The energy profiles of these polytypic transformations, as well as the geometry of all intermediate and transitional phases, are calculated. It is shown that all transition states have monoclinic symmetry.