Most materials exhibit positive thermal expansion and expand when heated, although a small number display negative thermal expansion and contract instead. If combined, these two types of material can form a composite that does not expand at all as the temperature is changed. Such ‘zero-expansion’ composites are useful because they can withstand rapid variations in temperature.

Now Mercouri Kanatzidis and colleagues at Michigan State University have discovered that a non-composite material made of ytterbium, gallium and germanium can also exhibit zero-expansion behaviour. Moreover, the new compound conducts electricity, whereas previous zero-expansion materials were insulators. Furthermore, the effect is observed over a wide temperature range - between 100 and 400 Kelvin.

Kanatzidis and co-workers speculate that as the sample cools, delocalized electrons in the valence band associated with the gallium atoms become localized on ytterbium atoms, which expand as they accept the electrons. The gallium atoms, on the other hand, contract. Since the gallium atoms only contract by a small amount, this leads to a positive thermal expansion coefficient in one direction. However, the material can be prepared and processed so that there is an almost equal and opposite contraction in the other two directions. This results in a negligible overall volume change in the unit cell.

“We hope that these results will allow us to look for zero-expansion materials among semiconductors and intermetallic compounds, which had not been thought of before now,” Kanatzidis told PhysicsWeb. “Perhaps new systems that take advantage of such valence transitions could be considered. This is a fresh approach to such materials.”