Structure factor of X4 tetrahedral molecular liquids: Competition between intramolecular and intermolecular atomic spacings

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Abstract
Structure factor Sm(Q) of liquids of simple tetrahedral molecules X4 has been calculated as a function of ζ, where ζ is a ratio of the intermolecular atomic spacing L to the intramolecular atomic spacing l, i.e., ζ=L/l, under constraints of a fixed preferred orientation between the nearest neighbors and a fixed packing fraction of uncorrelated molecules. It turned out that the Sm(Q) depends strongly on the ratio ζ. An important feature of the calculated Sm(Q)’s is an appearance of a new peak at low Q region and its rapid growth with increasing ζ. The characteristic features of the experimental Sm(Q)’s of liquids P4 and VCl4 can be reasonably reproduced by the present simple calculation with ζ=1.64 for liquid P4 and 1.14 for liquid VCl4. These values of ζ are simply interpreted by the molecular size and intramolecular atomic spacing of each molecule.