INFLUENCE OF ATOMIC RELAXATIONS ON THE STRUCTURAL-PROPERTIES OF SIC POLYTYPES FROM AB-INITIO CALCULATIONS

A systematic study of ground-state properties of cubic and hexagonal silicon carbide polytypes (3C-, 6H-, 4H-, and 2H-SiC) is reported using well converged density-functional calculations within the local-density approximation and norm-conserving, fully separable, soft, ab initio pseudopotentials. Equilibrium results are obtained for the lattice parameters, atomic positions, bond lengths and angles, cohesive energies, and the bulk modulus. The internal degrees of freedom, i.e., atomic relaxations, are fully taken into account. The results are discussed in comparison with experimental data. We derive trends with the hexagonality for the molecule volume and the energetic ordering of the polytypes. Driving forces for the polytypism and the atomic relaxations are discussed. © 1994 The American Physical Society.