Phase segregation in silicon carbide-carbon solid solutions from XRD and NMR studies

SiC-C solid solution powders have been analyzed by X-ray diffraction (XRD) and magic-angle spinning nuclear magnetic resonance (MAS NMR). The XRD data reveal a previously unreported phase separation of the as-synthesized material into two cubic silicon carbide phases characterized by slightly different lattice parameters [4.35287(9) and 4.35841(6) Å]. The ²⁹Si MAS NMR spectra of the powders after different heat treatments (annealing in a vacuum and high-pressure sintering) show that the small excess of carbon (< 1 at %) in the β-SiC crystal structure has a large influence on the ²⁹Si chemical shift resulting in a displacement of the ²⁹Si MAS NMR peak from -18.5 ppm to lower field. At least five nonequivalent silicon sites have been detected in Si_1-x-C_1+x: the solid solution formed by high-pressure sintering (4 GPa, 1800 °C) of the as-synthesized SiC-C powder. These sites are assigned to point (carbon antisite) defects in the cubic silicon carbide structure.