Weak bonding of carbon atoms at corner sites in titanium-carbide nanocrystals

Recent experiments suggest that carbon atoms are disfavored at corner sites in transition-metal carbide nanocrystals. We examine this issue theoretically within density-functional theory, by focusing on the 3×3×3 titanium-carbide nanocrystals: Ti14C13 and Ti13C14. The former species has metal atoms at the corners and is seen experimentally in great abundance, while the latter has carbon atoms at the corners and is not seen at all in the experiments. Our energetic and vibrational calculations indicate that carbon atoms are significantly more weakly bound to corner sites than are metal atoms. Possible consequences of these weakened corner-carbon bonds are discussed.