The main peak of the oxygen-oxygen structure factor $S_{\mathrm{OO}}\mathrm{}\left(Q\right)\mathrm{}$ 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 ${\mathrm{SiO}}_2,$ and that its position is determined by the nearest-neighbor separation of voids in the spatial distribution of oxygen atoms.

• Received 8 February 2000

DOI:https://doi.org/10.1103/PhysRevE.62.1427