Voids in the H-bonded network of water and their manifestation in the structure factor

The main peak of the oxygen-oxygen structure factor S(OO)(Q) of liquid water shows an unusual doublet structure. The low-Q feature of the doublet shifts appreciably with pressure, between limits which correspond to the position of the first peak in the low- and high-density forms of amorphous ice. This pressure dependence has been cited as evidence of polyamorphism in water. It is shown that this feature is analogous to the "prepeak," or "first-sharp-diffraction peak" which is well known in network-forming ionic liquids, like SiO2, and that its position is determined by the nearest-neighbor separation of voids in the spatial distribution of oxygen atoms.