Icosahedral order and defects in metallic liquids and glasses

Abstract

Molecular-dynamics simulations of 500 particles have been performed to study the icosahedral order and the defects in the supercooled liquid and glass states of metallic ${\mathrm{Mg}}_{3}$${\mathrm{Ca}}_{7}$. Not only are Frank-Kasper polyhedra and Bernal ``hole'' polyhedra detected, but also a variety of defective icosahedra. Especially, the number of the type of defective icosahedron defined by eight 1551 bonds, two 1661 bonds, and two 1441 bonds is much greater than the number of any kind of Frank-Kasper and Bernal polyhedra. This strongly supports the physical picture of liquids and glasses being a disordered, entangled array of +72\ifmmode^\circ\else\textdegree\fi{} and -72\ifmmode^\circ\else\textdegree\fi{} disclination lines in an icosahedral medium.