The Structure of Ice II

Abstract

The stability of the high pressure ice forms discovered by Tammann and Bridgman at very low temperatures and atmospheric pressure has made it possible to obtain x-ray powder diffraction photographs of these forms. The experimental procedure is very briefly discussed. An analysis of the ice II diffraction patterns leads to a side-centered orthorhombic cell with a=7.80A, b=4.50A, and c=5.56A. This cell contains eight molecules, has the symmetry of space group V5—C2221, and gives a value for the density of 1.21. The proposed arrangement of the hydrogen atoms indicates that the transition from ice I to ice II breaks up the water molecule, and that ice II is an ionic crystal. According to the model described the effect of a pressure of 2100 atmospheres in forming ice II is: (1) To decrease the packing volume of the oxygen ion approximately three percent; (2) To break down the open ice I structure by a slipping of the oxygen ion layers relative to each other; (3) To give a four-coordinated structure where each oxygen is surrounded by a badly distorted tetrahedron of oxygen ions.