Credit: © 2009 NPG

The majority of materials expand as they are heated and shrink as they are cooled because the heat energy increases the amplitude of atomic or molecular vibrations. Some materials, however, show the reverse and contract as they are heated. This phenomenon, known as negative thermal expansion, is limited to mainly inorganic framework materials and a small number of polymers.

Now, Leonard Barbour and colleagues from the University of Stellenbosch in South Africa have discovered1 extremely large thermal expansion — both positive and negative — in a simple molecular crystal. The dumb-bell-shaped diyn diol forms crystals in which the molecules are stacked at an angle. At 330 K, this angle is 51°, but as the crystal is cooled the angle increases, reaching 54.2° at 225 K. This means that the crystal contracts in the direction of the cell axis along which the molecules are stacked, and expands in the directions of the other two axes.

What is particularly unusual about this material is the size of the expansion and contraction. The expansion coefficient is between eight and twenty times as large as that for a typical material, whereas the contraction coefficients are between twice and ten times as large.