Electronic structures of and composition gaps among the ternary carbides Ti2MC

The electronic structure of the Ti2MC phases have been calculated in the framework of the full-potential augmented-plane-wave method under the general gradient approximation with M=Al, Si, P, S, Sn, Ga, In, Ge, As, Pb, and Tl.Among these all but M=Si, P, As give rise to stable compounds whose bonding is driven by Ti d-M p and Ti d-C p hybridizations. In general, Ti d-M p hybridizations are located just below the Fermi level and are weaker bonds than the Ti d-C p hybridizations, which are deeper in energy. As a function of the p-state filling of the M element the Ti d-M p bands are driven to deeper energy. A transition occurs in the column of P and As whose p atomic energy levels match exactly the C p ones. This explains why Ti2PC and Ti2AsC are not observed since P and As would link more favorably with C. Finally, the Ti d-S p bonds are at lower energy and are stiffer than the Ti d-C p bonds in the Ti2SC compound. This shows that this compound must behave differently from the other Ti2MC phases, as demonstrated by its elastic properties. In agreement with other previous studies, Ti2SiC is found to be intrinsically stable but is not observed because it is in competition with other compounds in the Ti-Si-C phase diagram.