Antifluorite ice

In Figure 3, the 0-H BCP of the antifluorite structure had a lower degree of covalent character, H rC) (defined by Equation (3)) than the ice X 0-H BCPs. This initially seems a bit surprising but can be explained by the increased metallic character (see Equation (3)) of antifluorite ice, since from Equation (1) and (2) the Laplacian must be positive (for metallic character), so the G r() contribution to Equation (1) must dominate V r() and so reduce the covalent character. In ice X the O—O interactions are very largely covalent in character, which was not the case in ice VIII. The metallic character of antifluorite ice can be seen in Figure 4 no results are presented for ice X since the maximum value of the metallicity (rb) was found to be 0.2, well below the threshold of unity required. The onset of metallic character happens to coincide with the point where curves (i.e to the left hand side of the minimum in two parabolic fits) for the calculated EV data for ice X and antifluorite structures. ... 

For ice X and the (fee) antifluorite structure the quantification of various aspects of their structural and chemical character and their dependence with pressure was found using AIM in a novel approach. Metallic character was found to be present in the antifluorite structure, but did not persist with increased pressure since the BCPs then fell within the pseudopotential core radii. In future studies on the antifluorite structure it will be necessary to replace the core with the true all electron distribution. In addition we present a hypothesis for the physical meaning of the O—O bonding interactions, namely that they indicate the onset of, soft phonon modes that are known to accompany structural changes. The fact that there are no O—O interactions in the antifluorite structure is consistent with this hypothesis, since to date there aren t any higher pressure phases of ice than antifluorite ice and so no pressure induced phase change can occur in this structure. Thus our hypothesis would explain why there are no O—O interactions in the antifluorite structure. 

Figure 1 The sites of the bond critical point (BCP) data shown for ice X and the antifluorite structure in (a) and (b) respectively.

Figure 2 The relationship between the lattice parameter L(in Angstroms), and pressure P with the same simple exponential relation for comparative purposes P=A exp(- BL) given for ice X and the antifluorite structure in (a) and (b) respectively, where in (a) A = 5.80x10, B =4.04 and correlation = 0.9999 and (b) A = 8.95x10, B = 3.41, correlation = 0.998,...

---