Traditionally crystallographers have assumed that crystals are made up from identical repeating units. Each of these unit cells contains atoms in various positions, and these are then stacked together in a regular way to create the crystal structure. However, in some crystals the unit cells are not all identical, and then the material is known as a disordered crystal. Sometimes this disorder can affect the properties of the crystal, changing the way it responds to variables like temperature and pressure.
For such materials the traditional crystallography methods for studying the structure can fail. By using pulsed neutrons, Alex Hannon from ISIS and his colleagues have managed to study how disorder affects the way that some types of crystal behave.
In particular they have been studying a type of cyanide crystal that is known to contract in one direction when it is heated. “On first inspection the crystal appears to be a simple structure, made up from straight chains of atoms in parallel,” says Hannon.
But using a neutron diffraction technique particularly suitable for studying disordered crystals, they were able to measure the distances between individual atoms and show that the structure was not as simple as it first appeared. Normally when a material is heated the atoms vibrate and cause the material to expand. But, when the cyanide crystals were heated Hannon and his colleagues observed the chains of atoms contracting, like muscles suffering from cramp – a process called negative thermal expansion. “The chains of atoms behaved like a skipping rope. The vibration produced a wave down the rope, which pulled the ends of the rope in so that they were closer together,” explains Hannon.
Next Hannon and his colleagues want to take a look at some more complex forms of cyanide, with twodimensional layered systems.
Negative thermal expansion occurs in a number of materials. By understanding how it works it may eventually be possible to create new materials where the expansion is controlled. “A material that hardly expands or contracts upon heating might be very useful,” says Hannon.
Prof. Alex Hannon (ISIS), S Hibble (University of Reading)
Research date: December 2002
Local structure: the realm of the chemist?, SJ Hibble, AC Hannon in: SJL Billinge and MF Thorpe (Eds.), From semiconductors to proteins: beyond the average structure, Kluwer Academic/Plenum Publishers, New York, 2002, p. 129.