LOCAL STRUCTURAL MOTIFS IN A QUENCHED MODEL MONATOMIC LIQUID

Amorphous packings have been generated from steepest-descent quenches applied to a monatomic liquid simulated by molecular dynamics. The mod el for the interatomic forces employed here provides both liquids and solids with a mean coordination as low as seven at low densities as we ll as the close-packed configurations with a mean coordination of twel ve at higher densities. The low-coordination packings generated from t he low-density liquid contain microcrystallites with a simple hexagona l structure which, with increasing density, are eventually suppressed in favor of microcrystallites with a body-centered-cubic (bcc) structu re. The abrupt growth of the bcc microcrystallites occurs at densities well below the density for the hexagonal-bcc crystal transition at ze ro temperature and constant volume. The high-coordination packings que nched from the high-density liquid contain truly amorphous clusters wh ich are neither crystalline nor icosahedral, but share attributes of b oth. The first result shows that microcrystalline clusters can be impo rtant structural elements of even simple amorphous substances. The sec ond result resolves the conflicting claims concerning the abundance or scarcity of icosahedra in the densest amorphous substances. Both of t hese results challenge widely held pictures of simple amorphous substa nces.