Die Packungsdichte der Atome in Modifikationen von Kohlenstoff und Silicium

Abstract

Abstract The crystal chemistry of carbon and silicon is discussed with respect to the packing density of atoms in their crystalline modifications. The reduced atomic volumes of metallic carbon as calculated from diamond, hexagonal diamond and buckminsterfullerene C60 agree within less than 0.2, indicating the correctness of the reducing procedure. A much lower value calculated from graphite, rhombohedral graphite and a low-temperature modification of buckminsterfullerene remains unexplained. Two proposed crystal structures of a cubic body-centered form of carbon appear unacceptable. The reduced atomic volumes of diamond-type silicon and metastable γ-Si are in good agreement. Their values, calculated from six allotropic modifications (Si I, II, XI, V, VI, VII, and Si X) at pressures up to ≈200 GPa, are represented by an empirical exponential function of pressure. From this function, an isothermal equation of state is derived including the packing density of the atoms.