Iron swells up when squeezed with hydrogen - discovery that iron hydride can exist under high pressure suggests that it may be present at the Earth's core

Science News, August 3, 1991 by Elizabeth Pennisi

Intense pressure usually makes a material shrink. But when three geophysicists recently subjected iron and hydrogen to a pressure of 35,000 atmospheres, their sample expanded by 17 percent.

This phenomenon, reported in the July 26 SCIENCE, also expands current thinking about the composition of the Earth's core. The discovery that iron hydride can exist under very high pressure adds weight to the suggestion that its presence may account for the lower-than-expected density of the core, say John V. Badding, Russell J. Hemley and H.K. Mao of the Carnegie Institution of Washington (D.C.).

From calculations of the thermodynamics required for the formation and stability of iron hydride within the Earth, the researchers conclude that the compound could result from water reacting with iron in the core. "There could be a large amount of hydrogen in the core," Hemley told SCIENCE NEWS.

"They have given us a totally new class of observations," comments geophysicist Raymond Jeanloz at the University of California, Berkeley. "If we can show there is hydrogen down there, then it has some serious implications for the earliest history and atmosphere of the Earth."

For their experiments, the researches first placed a small sample of iron into a diamond anvil cell, then filled the cell with hydrogen gas and squeezed the sealed sample between the tips of the anvil's two diamonds until the iron suddenly expanded. "It puffed up like a sponge absorbing water," says Hemley. At the same time, the sample's smooth surface became rough and grainy.

The team then shined synchrotron radiation through the diamonds to study the deformed iron's structure under pressures of up to 620,000 atmospheres.

Normally, iron atoms subjected to intense pressure form closely packed layers of hexagons. The atoms in one layer line up with those two layers away, creating a simple pattern of alternating alignments. But when hydrogen works its way into the metal lattice, the iron hexagons shift, says Hemley. One layer of iron atoms still lines up with the atoms two layers away, but the layers sandwiched in between match up with atoms four layers away.

The researchers think the hydrogen settles into octahedral spaces between the iron atoms, altering their bonds and causing the sample to swell. The iron sops up so much hydrogen that there is almost one hydrogen atom for every iron atom. "That's an awful lot of hydrogen," Hemley says. The added hydrogen makes the material much less dense.

The impetus for this work, he says, came from a side effect seen during an earlier study of hydrogen's behavior. The Carnegie researchers noticed that the stainless steel gaskets in their diamond anvil cell reacted with the hydrogen they had used. When forced by pressure into a metallic element, hydrogen can make the metal brittle, so the steel in the anvil tended to flake and crack, Hemley says. The team has begun replacing the stainless steel with rhenium, which seems to hold up better, he adds.