Local structural motifs in a quenched model monatomic liquid.

Abstract

Amorphous packings have been generated from steepest-descent quenches applied to a monatomic liquid simulated by molecular dynamics. The model for the interatomic forces employed here provides both liquids and solids with a mean coordination as low as seven at low densities as well as the close-packed configurations with a mean coordination of twelve at higher densities. The low-coordination packings generated from the low-density liquid contain microcrystallites with a simple hexagonal structure which, with increasing density, are eventually suppressed in favor of microcrystallites with a body-centered-cubic (bcc) structure. The abrupt growth of the bcc microcrystallites occurs at densities well below the density for the hexagonal-bcc crystal transition at zero temperature and constant volume. The high-coordination packings quenched from the high-density liquid contain truly amorphous clusters which are neither crystalline nor icosahedral, but share attributes of both. The first result shows that microcrystalline clusters can be important structural elements of even simple amorphous substances. The second result resolves the conflicting claims concerning the abundance or scarcity of icosahedra in the densest amorphous substances. Both of these results challenge widely held pictures of simple amorphous substances.