The Structure of Ice III

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 diffraction photographs of these forms. Since ice III is extracted from the press by taking it through the ice II region the precautions taken to avoid consequent misinterpretation of the ice III films are given. The analysis of the ice III diffraction patterns leads to a body‐centered orthorhombic structure, and a unit cell of dimensions a=10.20A, b=5.87A, and c=7.17A. The unit cell contains sixteen molecules, has the symmetry of space group V_h²⁶‐Ibam, and leads to a value for the density of 1.105 g per cc. Each oxygen ion is surrounded by a distorted tetrahedron of oxygen ions in a manner similar to that of ice I. An arrangement of the hydrogen atoms is proposed which makes the ice III lattice ionic. The volume decrease in the ice I—ice III transition is shown to be the result of a rearrangement of the oxygen ions. A simple transition mechanism is described, which is used to interpret some of the phenomena observed by Bridgman. The nature of the ice II—ice III transition is also discussed. A correlation between the latent heat of the solid‐solid reactions involving ice I, ice II, ice III and the H–O–H bond angles is suggested. Some remarks are made concerning the liquid‐solid reaction at high pressure. A brief description of some qualitative results obtained from the unanalyzed ice V and ice VI films is made. A summary of all the conclusions drawn from the high pressure ice investigations is made.