The relationship between crystal structure and related material properties is discussed for the common 3C, 6H, 4H, and 2H polytypes of SiC. The theoretical results are derived in the framework of well converged density‐functional calculations within the local‐density approximation and the pseudopotential‐plane‐wave approach. In the case of electronic excitations additionally quasiparticle corrections are included. The lattice‐dynamical properties of the noncubic polytypes are described within a bond‐charge model. We focus our attention on the actual atomic structures, the accompanying lattice vibrations, thermodynamical properties, properties of layered combinations of polytypes, optical spectra, and surface equilibrium structures. On the one hand, the influence of the polytype on the material properties is considered. On the other hand, indications for driving forces of the polytypism are extracted.
The spin configuration of isoelectronic vacancies surrounded by first-row atoms is studied within densityfunctional theory ͑DFT͒ using the local spin density approximation. Allowing for a symmetry break in the electronic system, a mixed-spin state is found to be lowest in energy. It is accompanied by a magnetization density with reduced symmetry indicating a magnetic Jahn-Teller effect. The DFT results are discussed in terms of many-body wave functions of a quantum-chemical treatment. Examples considered are neutral vacancies in diamond and cubic SiC.
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