A new translucent crystal, made from gold, zinc, and cyanide, does something very few materials do: Instead of shrinking under pressure, it expands.

Most ordinary materials contract when pressure is applied equally from all directions. The new crystal’s counterintuitive response to squeezing is the result of a spring-like arrangement of gold atoms nestled within its hexagonal structure. As the springs compress, the crystal grows longer, increasing its length by as much as 10 percent – a change that’s actually visible when scientists put a chunk of the material under a microscope.

“We were stumped for a while as to why the effect was so strong for this material, until we noticed the little atomic-scale springs," said Andrew Goodwin, a chemist at the University of Oxford. "These help absorb the 'shock' of the pressure, and so allow the material to deform much more than others would."

Goodwin and his colleagues described the crystal in Nature Materials earlier this year; graduate student Andrew Cairns reported it yesterday at the American Crystallographic Association’s meeting in Hawaii.

Materials and systems that expand under pressure are not unknown to science. In fact, they're found in certain types of muscle, such as those that propel octopi and squid through water and curl an elephant’s trunk. But it’s only been in the last decade or so that scientists have been able to make such materials in the lab. The key involves creating a shapeshifting structure that can reorganize its atomic building blocks without falling apart.

"It's the material structure or architecture, rather than the composition, that drives the behavior," said Karena Chapman, a chemist at Argonne National Lab whose team recently described a different material that also expands under pressure.

A Cairns and A Goodwin, University of Oxford )

To make the new crystal, scientists mixed two salts in solution, one containing gold atoms; the other, zinc. When combined, the salts produce a translucent crystal called zinc dicyanoaurate. The crystal’s atomic structure resembles a lattice of six-pointed hexagons, with zinc atoms at the vertices and gold atoms flanked by cyanide molecules (a carbon atom bound to a nitrogen atom) in between.

Connecting the hexagons is the helical gold spring that helps absorb the applied pressure.

To test the crystal’s response to pressure, the scientists used a diamond-anvil cell, an apparatus that squeezes tiny bits of material between two diamonds. When the scientists began compressing their new crystals, the crystals began to expand.

At 1 gigapascal – a pressure at which most materials have already shrunk by 2 or 3 percent – the crystal had expanded by 5 percent. At 10 gigapascals, about 100 times more than the crushing pressure at the bottom of the Mariana Trench, the deepest spot in the sea, the crystals were still growing.

“What surprised us was the magnitude of the response,” Goodwin said. “We had a reasonably good idea that we would get to see it expanding under pressure - we had been designing similar materials for the last few years, including ones that shrink when heated.”

What good is such a weird material? It could make existing pressure sensors ten times more sensitive, Goodwin says. Or, the unshrinkable crystals could be used in smart materials for controlling circuits or directing beams of light. Maybe someday, the crystals could be used to create an artificial squid-like muscle – one that responds to pressure, rather than electrical signals, as ours do.

"One just looks forward to the even more extreme behaviors that will be engineered in the next generation of materials," Chapman said.