Extended Hückel tight-binding band structure calculations are used to address the chemical bonding and elastic properties of Zr 2 AC (A=Al, Si, P, and S). Elastic properties are interpreted by analyzing the density of states and the crystal orbital overlap population for the respective phases. Our results show that the bulk modulus of these ternary compounds is determined by the strength of Zr-A bonds.

The electronic structure and elastic properties of the 4d transition metal carbides MC (M = Y, Zr, Nb, Rh) were studied by means of extended Hückel tight-binding band electronic structure calculations. As the valence electron population of M increases, the bulk modulus of the MC compounds in the rocksalt structure does not increase monotonically. The dominant covalent bonding in these compounds is found to be M-C bonding, which mainly arises from the interaction between M 4d and C 2p orbitals. The bonding characteristics between M and C atoms affecting the variation of the bulk modulus can be understood on the basis of their electronic structure. The increasing bulk modulus from YC to NbC is associated with stronger interactions between M 4d and C 2p orbitals and the successive filling of M 4d-C 2p bonding states. The decreased bulk modulus for RhC is related to the partial occupation of Rh-C antibonding states.

The orthorhombic MgB2C2 structure contains well-separated parallel graphite-like B2C2 2-layers which extend infinitely in two dimensions. Three possible ways to distribute B and C atoms in the hexagonal sublattice sites are adopted. Band structures for the hypothetical distribution patterns are examined to assess the electronic stability of these phases and to account for the observed arrangement by means of extended Hückel tight-binding calculations. The preferred choice is the layer with B and C alternating strictly so that B is nearest neighbor to C and vice versa. A rationale for this is given. Due to the alternation of B and C within the honeycomb layers, MgB2C2 is a band insulator, which through partial substitution of Mg with Li, is predicted to turn metallic with holes in the σ bands at the Fermi level.

ABSTRACT. The chemical bonding and elastic properties of Ti2AC (A = Ga, Tl) have been investigated by means of extended Hückel tight-binding band structure calculations. The bulk modulus of Ti2AC decreases as Ga is replaced with Tl at the A sites. This can be understood by considering the relative strength of Ti−A bonds resulting from the different atomic size of 3A-group elements. The analysis of the projected density of states (PDOS) and the crystal orbital overlap population (COOP) for the respective phases shows that Ti−Ga bonds in Ti2GaC are stronger than Ti−Tl bonds in Ti2TlC.

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