Phonon instability in nanocrystalline silicon carbide

Abstract

PACS 61.46. Fg, 63.22.+ m In recent experiments by a low-pressure, low-temperature technique, stable nanotubules of rocksalt SiC were synthetized in massive amounts. Ab-initio perturbative density-functional theory calculations in the bulk rocksalt phase provide evidence of drastic phonon softening at a critical pressure of about 2 GPa. To explain the exceptional persistence of the rocksalt structure at ambient conditions we propose that the high pressure induced by interfacial curvature may quench the phonon instability in the nanocrystals down to this minimum threshold, corresponding to the observed nanocrystal size of about 2 nm. Below such pressure the onset of mechanical instability forbids further growth and transformation to ordinary SiC. Nanocrystalline rocksalt SiC is a remarkable example of a material whose unstable, high-pressure phase is stabilized at ambient conditions upon reducing the grain size in the nanometre range.