References: p. 279 (8 tit.)

During several decades the question of the presence or absence of Laplace compression in particles less than 0.1 microns, has been discussed. A consequence of this compression could be an increase of X-ray density in small particles. The study of the structure of final products of wire electrical explosion (ultrafine powders, UFP) has shown that their X-ray density is lower than for metals in a stable massive condition. It is shown that the metals (Al, Cu), not having polymorphic modifications are characterized by a specific defect type-static displacement of atoms concerning X-ray density, lowered up to 0.1-0.2%. In the case of polymorphism of UFP metals, the crystal modifications with lowered density are stabilized: iron as γ-Fe, tungsten as β-W. As regards formation of chemical compounds, oxide UFPs are also formed with lowered X-ray density (γ-Al2O3 , TiO2)