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.

Integral equation theory for the four bonding sites model of I. Structure factor and compressibility associating fluids

“…We note that the above relations amongst the densities are identical to those obtained in the framework of thermodynamic perturbation theory [2,26,32]. In particular, using equations (30) and (31) together with equations (37)± (39), we have the following relations for X G (1) and X F (1):…”

“…Partial densities Within the ideal network approximation the relationships (30) and (31) together with equations (6)± (9) allow us to write a`polymer'-like relation for the captured densities: (35) .…”

“…Our other purpose is to study the structure factor at small wave numbers and to detect the role of association in the formation of the so-called pre-peak [30,31].…”

The Multi-Density Integral Equation Approach in the Theory of Complex Liquids

“…The possibility of the existence of a prepeak in the structure factor of water is discussed based on the combination of computer simulation and integral equation theory [45]. Lately, such a peculiarity has been detected in many polymer and glassing substances and in ionic liquids [46], where, due to the correlations between relatively large clusters, the ordering in mesoscopic scale can appear.…”

Slow dynamic properties of molten silver halides

“…Those differences result from the fact that bonding in the silver halides occupies an intermediate position between typically ionic materials and fully covalently bonded ones. There are some differences among silver halides in the liquid state, and the structural, transport and dynamic properties of molten salt systems have been studied by many workers [2][3][4][5][6][7]. The local structural unit of AgI is known to be a tetrahedral in the solid state below the melting temperature and this local order is still preserved even in the liquid state [4,7].…”