Density functional calculations of the electronic structure and optical properties of the ternary carbides Al4SiC4 and Al4Si2C5

The electronic structure and spectroscopic properties of two ternary aluminum silicon carbide ceramics Al4SiC4 and Al4Si2C5 are studied by density functional theory calculations based on the orthogonalized linear combination of atomic orbitals method. Both crystals are shown to be small gap semiconductors with indirect band gaps of 1.05 and 1.02 eV, respectively. The calculated hole and electron effective mass and the interband optical properties, in the form of the complex dielectric function, show a high degree of anisotropy which can be traced to the unique structures of these two crystals. The calculated refractive indices are consistent with the values proposed in the literature. Mulliken effective charge and bond order calculations show that these crystals have a high degree of covalency with considerable charge transfer from Al and Si to the C atoms. The x-ray absorption near-edge-structure for all crystallographically nonequivalent sites (K and L edges) is calculated and compared with those of cubic SiC. It is shown that the site-averaged Si-K and Si-L-3 edges, and also the C-K edges are slightly different and broader than those of cubic SiC. Potential applications of these ternary ceramics are also discussed.