Abstract
A detailed and systematic density-functional theory (DFT) study of a series ofearly transition-metal carbides (TMCs) in the NaCl structure is presented. Thefocus is on the trends in the electronic structure and nature of bonding, whichare essential for the understanding of the reactivity of TMCs. The employedapproach is based on a thorough complementary analysis of the electron densitydifferences, the density of states (DOS), the band structure and the real-spacewavefunctions to gain an insight into the bonding of this class of materials and get a moredetailed picture of it than previously achieved, as the trend study allows for asystematic identification of the bond character along the different bands. Our approachconfirms the presence of both the well-known TM–C and TM–TM bonds and, moreimportantly, it shows the existence and significance of direct C–C bonds in allinvestigated TMCs, which are frequently neglected but have been identified in somecases (Zhang et al 2002 Solid State Commun. 121 411; Ruberto et al 2007 Phys.Rev. B 75 235438). New information on the spatial extent of the bonds, theirk-space location within the band structure and their importance for the bulk cohesion isprovided. Trends in covalency and ionicity are presented. The resulting electron-structuraltrends are analyzed and discussed within a two-level model.
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