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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.

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We report first-principles calculations of the structural, lattice-dynamical, and dielectric properties for zincblende and wurtzite BN and AlN. The ground-state properties, i.e., the lattice constants, the bulk moduli, the ionicity factors of the chemical bonds, and the elastic constants, are calculated using a plane-wavepseudopotential method within the density-functional theory. A linear-response approach to the densityfunctional theory is used to derive the Born effective charges, the high-frequency dielectric constants, and the phonon frequencies and eigenvectors. The different behavior of the structural and lattice-dynamical properties of BN and AlN is discussed in terms of the different ionicities, strengths of the covalent bonds, and the atomic masses. Our results are in excellent agreement with the experimental data available.

We present a first-principles study on the pressure-dependent properties of cubic and hexagonal polytypes of silicon carbide ͑SiC͒. Our calculations have been performed within density-functional perturbation theory, using the plane-wave pseudopotential approach. The stability of several high-pressure SiC phases is discussed in terms of the ionicity and metallicity of the Si-C bonds. Furthermore, we investigate pressure dependence of the zone-center frequencies, of the Born effective charges, and of the static and high-frequency dielectric constants for 3C, 2H, and 4H polytypes of SiC. Whereas the structural and electronic properties of the cubic and hexagonal polytypes are very similar, remarkable pressure-induced differences are found for the dynamical and dielectric properties. The unusual behavior of the transverse effective charge recently observed experimentally for 6H SiC is discussed. ͓S0163-1829͑96͒02119-4͔

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We present first-principles calculations of the structural, lattice dynamical, and thermal properties as well as Raman results for cubic silicon carbide (3C Sic). The plane-wave pseudopotential approach to density functional theory (DET) in the local density approximation has been used to calculate the equilibrium properties of 3C Sic, i.e., the ground-state energy, the band structure, the valence electron density, the lattice constant, the bulk modulus, its pressure derivative, and the ionicity factor of the chemical bonds. The linear-response theory within DET has been used to obtain the phonon frequencies, the eigenvectors, and the mean-square atomic displacements. Furthermore, we calculated the mode Gruneisen parameters, the internal-strain parameter, the elastic constants, the Born effective charge, and the high-frequency dielectric constant. The specific heat at constant volume and at constant pressure, the thermal expansion coefficient, the temperature dependence of the lattice constant, and that of the isothermal and adiabatic bulk modulus have been derived within the quasi-harmonic approximation. Finally, the second-order Raman spectrum of 3C Sic has been calculated using phenomenological polarizability coefficients and ab initio frequencies and eigenvectors. 0 1995 John Wiley Kr Sons, Inc.