Mechanism of the 2H → 6H solid state transformation in SiC

Single crystals of 2H SiC transform directly into the 6H (ABCACB) structure when the transformation nucleates at temperatures above 2000°C. The 2H close-packed structure may be transformed to the 6H structure by displacing every third layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H to 6H structural transformation by such a layer displacement mechanism has been developed. The fact that all the observed solid state transformations in SiC crystals commence with the random insertion of stacking faults and then proceed further to create a statistically ordered structure, permits such a theory to be developed. Exact expressions for the diffracted intensity from such crystals have been obtained and the different diffraction effects observable on single crystal X-ray photographs predicted. A comparison of the theoretically predicted diffraction effects with those visible on the X-ray photographs of SiC crystals undergoing the 2H to 6H transformation shows that the structural transformations in SiC occur by the layer displacement mechanism.