Short-range order of liquid and undercooled metals:: Ab initio molecular dynamics study

We present a series of ab initio molecular dynamics simulations (MD) for liquid Ni, Zr and Ta to investigate the local structure and its evolution upon undercooling, and we analyze our results with respect to recent neutron scattering experiments. By means of the common-neighbor analysis, which gives a three-dimensional image of the inherent structures, we have found an evolution upon undercooling that depends on the system under consideration. The short-range order (SRO) of Ni is characterized by icosahedral and distorted icosahedral structures, and undercooling is accompanied by the occurrence of a complex polytetrahedral SRO that consists of more numerous Frank and Kasper polyhedra larger than the simple icosahedron. The same holds for Zr but we show that a strong competition exists with the bcc-like SRO which increases with the degree of undercooling. The SRO of Ta shows a complex polytetrahedral type order, very close to that of the A 15 phase upon undercooling. The time evolution of the configurations is also examined in terms of the mean-square displacement. (c) 2007 Elsevier B.V. All rights reserved.