Signs of altered bonds in squeezed ice.

The hydrogen bond-an attraction between a hydrogen atom in one molecule and an atom belonging to another molecule-plays a crucial role in determining the characteristics of water and the microscopic structure of ice.

Theorists have predicted that a sufficiently high pressure can alter the bonding in ice, transforming the material from a molecular solid in which hydrogen bonds hold water molecules in place into an ionic material made up of oxygen and hydrogen ions (protons). In this transformation, the strong, covalent bonds that normally link two hydrogen atoms to a single oxygen atom in a water molecule would become indistinguishable from the hydrogen bonds that link neighboring molecules.

Now, a team of researchers has obtained experimental evidence that for the first time pinpoints the transition in ice from covalent molecular bonding to complete hydrogen bonding. Alexander F. Goncharov and his colleagues at the Carnegie Institution of Washington's Center for High-Pressure Research in Washington, D.C., report their findings in the July 12 Science. Goncharov and his coworkers loaded ordinary ice into a small hole in a stainless steel gasket between two diamonds. They pushed the diamonds together to squeeze the sample (SN: 7/6/96, p. 6).

At pressures greater than 2 gigapascals (20,000 times atmospheric pressure), water exists in a form known as ice VII. In this structure, the oxygen atom of each water molecule is hydrogen-bonded to four of the water molecule's eight nearest neighbors, producing a tetrahedral arrangement.

At 60 GPa, measurements of the infrared light reflected by the compressed ice indicated a distinct change in the vibrations of water molecules. The observed transition "provides evidence for symmetric hydrogen-bonded states in ice," the researchers conclude.

They also found a similar transition, at a pressure of 70 GPa, in the ice phase of heavy water, in which deuterium atoms replace hydrogen atoms.