Size Changes of Lattice Parameters in Ultradisperse Diamond and Silicon

A relative increase of the lattice parameters equal to 2.05 × 10⁻³ ± 1.8 × 10⁻⁴ is established by X-ray analysis in ultradisperse powder diamond with average size of crystallites equal to 4.6 ± 0.4 nm as compared with the lattice parameter of a large-crystal sample. The effect found for diamond as well as the experimental data of the increase of the lattice parameters in ultradisperse samples of silicon established previously are explained by the size change of intracrystalline pressure stipulated by the interaction of the excess negative charges arranged on the bond lines CC (or SiSi), and the positive cores of the atoms of the covalent crystal. The size dependences of the relative change of the lattice parameters are calculated in the frameworks of the point model of the charge lattice for diamond and silicon. The dependences show a monotonous increase of the lattice parameters when decreasing the sizes of the crystals. Calculated dependences are in good agreement with experimental data.