  Three new phases of ternary transitional metal carbides, namely MAX phases,
Nb4SiC3, Nb4GeC3 and Nb4GaC3 were proposed by means of density functional
theory, and they were all predicted to be mechanically stable. It was figured
out that the alpha type crystal structures of the hypothesized nanolaminates
are energetically favored than beta type. Together with the known Nb4AlC3, the
role of A group elements in Nb4AC3 (A= Al, Si, Ga, Ge) on various properties
were systematically investigated. Not only bulk moduli, compressibility, shear
moduli, and Youngs moduli, but the approach to the ductility was discussed and
raised as that the ductility can be achieved by forming Nb4AC3 containing more
heavier p electrons. All the studied Nb4AC3 phases exhibit the metallic nature
that results from the Nb-4d states being dominant at the Fermi level. The
typical 4d-2p hybridization between Nb-C leads to strong covalent bond and the
contribution from the weaker 4d-3p(4p) hybridization between Nb-A are also
identified. The p electrons in A elements can effectively alter the covalency
and the iconicity of the bonds that govern the compressibility, ductility and
even superconductive properties. The extremely high population at the Fermi
level in the density of states and the nesting effects in the Fermi surface
shown in Nb4SiC3 both bring this material to a potential superconductor. The
regular anisotropic behavior in the dielectric function is present but not in
Nb4SiC3. Furthermore, Nb4GeC3 was also predicted to be a very promising
candidate solar heating barrier material. The present work gives insight to the
role of A elements on the mechanical, superconducting and optical properties of
Nb4AC3 phases, and the obtained tendency by changing A elements will fertilize
the desirable materials design of functional ceramics.
