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The semi‐empirical tight‐binding (TB) method within the sp3s* basis has been used to investigate the electronic structure and optical properties of silicon carbide (SiC) polytypes. A TB model for the electronic structure calculations of the (4H, 6H and 8H) SiC polytypes is presented. The procedure involves the construction of the TB Hamiltonian of the (4H, 6H and 8H) SiC polytypes from the wurtzite one. The set of TB parameters is transferable to all hexagonal structures, which overcomes the need for extensive experimental data. The band structures for different SiC polytypes are used to determine the electron effective masses. The results are shown to be consistent with available experimental data and other theoretical calculations. Furthermore, the joint densities of states are calculated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the magnetic and optical properties of various combinations of ordered spin state configurations between low-spin (LS) state, intermediate-spin (IS) state, and high-spin (HS) state of LaCoO(3) . In this study, we use the state-of-the-art first principles calculations based on generalized gradient (GGA) + Hubbard U approach. The excited-state properties of different spin configurations of LaCoO(3) such as the X-ray absorption spectra, optical conductivity, reflectivity, and electron energy loss are calculated. We have demonstrated that the optical spectra results can be used for analyzing the spin state of Co(3+) ion. The first specie is the local excitation of IS cobalt ions in the LS ground state. The second excitation leads to the stabilization of the mixed IS/HS Co(3+) metallic state. At low temperature, the comparison between O 2p and Co 3d projected density of states with the experimental valence band spectra indicates significant IS Co(3+) ions and this is in sharp contrast to the HS state which is negligible. The line shape of O 2s and Co 3d core level spectra are well reproduced in this study. The present results are in excellent agreement with the available experimental data. The variation in the spectra of different configurations of LaCoO(3) suggests a changing in the spin state as the temperature is enhanced from 90 to 500 K.