Most materials contract when they are cooled and get bigger when they are warmed up. But some substances do the opposite by shrinking in certain directions as they are heated and expanding when cooled. Now researchers in the UK have found an inorganic crystalline material composed of silver, cobalt, carbon and nitrogen that expands more than any other known material when cooled.

Negative thermal expansion occurs when, during heating, a crystal contracts along one or more crystal axes, while positive thermal expansion describes what happens when the material expands along one or more of these axes. Using X-ray diffraction Andrew Goodwin, a physicist at the University of Cambridge and colleagues at the Rutherford Appleton Laboratory and Durham University, found that silver(I)hexacyanocobaltate(III), or Ag3Co(CN)6, has a coefficient of thermal expansion of around –120 × 10–6 K–1 in one direction, some 14 times larger than the previous best material, ZrW2O8 (Science 319 794).

Colossal expansion

Moreover, the material expands so much along the other axes when heated that its coefficient of thermal expansion in this direction is 140 × 10–6 K–1, some 10 times bigger than any other material. The researchers use the term ‘colossal’ to describe a material that has an axis with a thermal expansion more than 100 × 10–6 K–1, meaning that Ag3CoC6 is the first material with both colossal positive and negative thermal expansion.

You can imagine the silver framework like a collapsible garden fence Andrew Goodwin, University of Cambridge

The material displays this unusual behaviour because of its sandwich-like structure in which Co(CN)6 octahedra sit between layers of silver atoms in the middle. As the bonds between the silver atoms are relatively weak, the silver layers can flex easily. “You can imagine the silver framework like a collapsible garden fence” says Goodwin. As the material is cooled this flexing of the silver layer allows it to contract which then pushes the octahedra of the remaining surrounding ions out and causes the material to stretch when cooled.

The researchers think that the material could be used to stabilize mirrors and other optical devices in satellites which can be very sensitive to changes in dimension. Such devices change position because the satellites heat up and cool down so much as they orbit the earth. Goodwin and colleagues believe that applying a thin coat of the material to the mirror could counteract this change of dimension and avoid the need for it to be mechanically adjusted.