The role of group III, IV elements in NbAC MAX phases (A = Al, Si, Ga, Ge) and the unusual anisotropic behavior of the electronic and optical properties.

Niobium based NbAlC, NbSiC, NbGeC and NbGaC were investigated by means of density functional theory. Together with the known NbAlC, the role of group III, IV elements in various properties of NbAC (A = Al, Si, Ga, Ge) was systematically investigated, and particularly the bulk moduli, shear moduli, and Young's moduli helped us to approach the ductility. All the studied compounds were found to be mechanically stable, and they also exhibit the metallic nature that results from the Nb-4d states being dominant at the Fermi level. The typical 4d-2p hybridization leads to strong Nb-C covalent bonding and a relatively weaker 4d-3p (4p) hybridization between Nb and A is identified. The latter does perturb the performance of materials. By varying A elements in NbAC, the position and the width of the p states as well as hybridizations are altered, which determine the covalency and the ionicity of the chemical bonds. A high density of states at the Fermi level and the nesting effects in the Fermi surface are identified in NbSiC and linked to its unusual anisotropic behavior. Furthermore, NbGeC is predicted to be a very promising candidate solar heating barrier material. Overall, the present work gives insights into the role of A elements in the electronic structure and the physical properties of NbAC compounds. The tendencies and rules established here will help in the designing of functional ceramic materials with desirable properties.