Structure of covalent liquids

Abstract

Bond directionality and network formation from local structural units are the signature of covalent bonding. On melting a 3D network of covalent bonds tends to break into a metallic liquid (e.g. in Si, Ge and GaAs), unless a sufficiently large electronegativity difference between the components stabilizes the electronic structure through chemical short-range order. The melt may then be a semimetal (e.g. Li4Pb and KPb), an ionic semiconductor (e.g. CsAu) or an insulator (e.g. ZnCl2). Bonding appears to be more stable in networks of lower dimensionality (D=2 as in GeSe2 and YCl3, D=1 as in Se and BeCl2, and D=0 as in P, SbCl3 and AlBr3). Melting from D=2 to D=0 occurs in AlCl3. Intermediate-range order may be preserved in the melt through interatomic correlations over distances of order 5-10 AA. The experimental evidence on illustrative examples of these various trends is reviewed, with emphasis on the interconnection between stable local coordination and intermediate-range order. Parallel illustrations are given of results from simulations based on empirical potentials or fully quantal methods, from data analysis based on the Reverse Monte Carlo method and from primitive models amenable to integral-equations techniques.