If you squeeze a normal object in all directions, it shrinks in all directions. But a few strange materials will actually grow in one dimension when compressed. A team of chemists has now discovered a structure that takes this property to a new level, expanding more dramatically under pressure than any other known material. The finding could lead to new kinds of pressure sensors¹ and artificial muscles. Andrew Cairns, a graduate student at the University of Oxford³ and a member of the research team, will discuss the new material and its applications at the American Crystallographic Association meeting held July 20-24 in Honolulu.

 

Negative linear compression, or NLC, has existed for millions of years; in fact, biologists believe octopi（章鱼） and squid（鱿鱼） use the phenomenon to make their muscles contract. Only in recent decades, however, have scientists learned to design materials with this property. Until a few years ago, none of these humanmade structures had been found to expand more than a fraction of a percent under compression, making them of limited use in engineering. But researchers are now learning how to design materials that expand far more than those previously⁴ known. The trick, say the scientists presenting this latest work, is to look for structures that can respond to pressure by rearranging their atoms in space without collapsing⁵.

 

The material the research team discovered, zinc⁶ dicyanoaurate, does just that. Its unique structure combines a spring-like helical（螺旋形的） chain of gold atoms embedded⁷ in a honeycomb-like framework made of gold, cyanide (carbon bonded⁸ to nitrogen), and zinc. When the chain is compressed, the honeycomb flexes⁹ outward by as much as 10% -- several times what had been achieved by any previous material. The scientists call this large response "giant negative linear compressibility," and compare it to a collapsible wine rack that folds up horizontally by expanding substantially in the vertical¹⁰ direction. Andrew Goodwin of Oxford, leader of the research team, says these wine rack structures represent "a new block in our Lego kit¹¹."

 

Zinc dicyanoaurate's unique properties make it promising¹² for several applications. In the immediate¹³ term, the material, which is transparent¹⁴, could be used as an optical pressure sensor². Compression causes the crystal spacing to narrow in one direction and widen in another, changing the path light takes through the material in a way that is sensitive to tiny variations in pressure. A longer-term application is artificial muscle design. Our muscles contract in response to an electric field, but new muscles could be designed to contract when pressure is applied¹⁵, as biologists believe octopus¹⁶ muscles do.

 

Goodwin's team is now working to understand more fully¹⁷ the mechanisms¹⁸ behind NLC. But even without a complete picture of nature's design principles, they feel confident zinc dicyanoaurate is already "pushing the limits" of how far any material will be able to expand under pressure. "We've got a pretty good feel for what the limits are," Goodwin says. "This material is pretty special."

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