Voids in network-forming liquids and their influence on the structure and dynamics

A Voronoi analysis is applied to clarify the origin of certain unusual physical properties of three representative network-forming ionic liquids: SiO2, ZnCl2, and BeCl2. In each of these fluids, the local structure under ambient pressure comprises a tetrahedral unit of anions around cations which link together to form a network. The Voronoi analysis is used to characterize the empty space (voids) within the network. Despite sharing the same local tetrahedral arrangement, the network properties of the three liquids differ markedly on an intermediate length scale and this leads to significant differences in the properties of the voids. It is shown how the void analysis helps to interpret the anomalous behaviour of the diffusivity in SiO2, which over a certain range of density increases with increasing density, and to clarify the origin of the intermediate range order in the atomic positions, as seen in the much discussed first sharp diffraction peak (or pirepeak) in diffraction data

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

Structural and thermodynamic properties of different phases of supercooled liquid water.

Computer simulation results are reported for a realistic polarizable potential model of water in the supercooled region. Three states, corresponding to the low density amorphous ice, high density amorphous ice, and very high density amorphous ice phases are chosen for the analyses. These states are located close to the liquid-liquid coexistence lines already shown to exist for the considered model. Thermodynamic and structural quantities are calculated, in order to characterize the properties of the three phases. The results point out the increasing relevance of the interstitial neighbors, which clearly appear in going from the low to the very high density amorphous phases. The interstitial neighbors are found to be, at the same time, also distant neighbors along the hydrogen bonded network of the molecules. The role of these interstitial neighbors has been discussed in connection with the interpretation of recent neutron scattering measurements. The structural properties of the systems are characterized by looking at the angular distribution of neighboring molecules, volume and face area distribution of the Voronoi polyhedra, and order parameters. The cumulative analysis of all the corresponding results confirms the assumption that a close similarity between the structural arrangement of molecules in the three explored amorphous phases and that of the ice polymorphs I(h), III, and VI exists