Phase Segregation in Silicon Carbide−Carbon Solid Solutions from XRD and NMR Studies

Abstract

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 Si1-xC1+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.