Vacancy ordering and medium-range structure in a simple monatomic liquid

We analyze the origin of medium-range structural order in a simple monatomi c liquid simulated by molecular dynamics. Convincing evidence is presented that this structural order, manifested by a prepeak in the structure factor , can be interpreted as chemical ordering in a mixture of atoms and atom-si ze vacancies. It is concluded that the liquid is able to sustain the presen ce of vacancies due to its predominantly icosahedral local order. The prese nt result thus indicates that vacancy-induced medium-range order should be a common feature of icosahedrally ordered simple liquids, e.g., simple meta llic glass formers. [S0163-1829(99)08001-7].