“Bond”-Orientational Short Range Order in Liquid Aluminium

Abstract

With the suggestion of D. Nelson et al. of some kind of orientational long range order underlying Glass Formation (v., e. g., ref. 1 and references therein), there has been an increasing interest also in the finite range correlation between the spatial orientations of pairs of nearest neighbours in undercooled liquids, and even in their short-range correlations in equilibrium liquids (above the melting point). The latter are interesting in their own right, as the easily measurable and calculable pair correlation function, giving only the distribution of atom pair distances, is unable to distinguish, e. g., between the different ways in which the twelve nearest neighbours of an atom are distributed over a spherical shell with the mean nearest neighbour distance as its (approximate) radius. This is apparent from the three examples given in Fig. 1 of ref. 1, viz., fcc, hcp, and ico-sahedral close packings around a central atom. In the terminology of ref.1, the pair distribution function only gives the length distribution, not the orientational correlation of these twelve nearest neighbour “bonds”, a “bond” being defined as a pair of atoms with a distance falling within the first peak of the pair the correlation function.