Logic dictates that when you increase the pressure acting on a material, it should compact. So a report from an international team of scientists that they have discovered a crystal formation that expands under pressure is intriguing. The counterintuitive behavior may be exploited to make a crystal sponge for chemical cleanup.

Writing in the December 19 issue of the Journal of the American Chemical Society, the researchers describe the behavior of natrolite, a type of zeolite, under increasing pressure. Zeolites are solids characterized by a three-dimensional structure containing regularly spaced pores within the molecular framework of atoms that includes aluminum, silicon and oxygen (see top image). When the scientists subjected natrolite to pressures up to 50,000 times the normal atmospheric pressure between two diamonds, the material initially compressed, as expected. But when the pressure ranged between 8,000 and 15,000 times atmospheric pressure, the crystal expanded (bottom image). "This is not supposed to happen," says co-author Thomas Vogt of Brookhaven National Laboratory. "Normally, when you squeeze something, it's supposed to get smaller. This stuff gets bigger." As the pressure increased, the material compressed further.

An x-ray analysis suggests that the material expanded because extra water molecules were squeezed into the pores within the natrolite. Terming the unusual property "pressure-induced expansion," the team suggests that the material may be used to mop up chemical or radioactive pollutants. "When you increase the pressure and the material gets bigger, the pores get bigger, too," explains co-author Joseph Hriljac of the University of Birmingham. If pollutant molecules enter the structure, he says, "when you release the pressure, the pore would get smaller and trap the pollutants inside."