Structure of Ice III BY BARCLAY KAMB AND ANAND PRAKASH

Ice III, least dense of the high pressure forms of ice (0c= 1.160 g.cm-3 at -175°C, 1 atm), has a tetragonal structure (P41212, a= 6.73, c= 6.83/~,) involving a tetrahedrally linked H-bond framework with mean bond length 2.775 A.. The increase in density relative to ice I (0-94 g.cm-3) is accomplished by distortions from ideal tetrahedral coordination which allow two or three non-bonded neighbors to approach each water molecule to distances of about 3.6 A, much closer than the corresponding distance of 4.50/~ in ice I. The ice III framework has numerous five-ring connections, whereas only six-rings (and larger) are present in ice I; it is topologically analogous to the framework in silica-K, but corresponding structural parameters in the two analogs differ markedly. One O • • • O • • • O coordination angle in ice III has the extraordinarily large value 143 °. This should cause a marked destabilization of the water molecule orientation presented into this angle, and could lead to proton ordering at lower temperatures, for which there is some independent physical evidence. The X-ray data refine to R= 0.051 for a half-hydrogen (disordered) model, and to R = 0.057 for one of the four full-hydrogen (ordered) proton arrangements possible in P41212. Although the difference between these results is not statistically significant, the crystallographic evidence is weighted in favor of the half-hydrogen model. The protonordered arrangement admitted as an alternative by the X-ray data utilizes the most favorable watermolecule orientations available in the structure.