Whilst most materials expand when they are heated, some are found to contract as the temperature is increased. These negative thermal expansion (NTE) materials are of technological importance for their ability to counter the positive thermal expansion (PTE) of ‘normal’ materials. Neutron diffraction has been used to show that silver hexacyanocobaltate, Ag3[Co(CN)6], exhibits ‘colossal’ NTE – an effect ten times stronger than the normal PTE of engineering materials. The unusual properties of Ag3[Co(CN)6] stem largely from the incredible flexibility of its atomic-level structure. Assembled from Co–C–N–Ag–N–C–Co covalent linkages, the framework lattice is free to flex with essentially no energy cost. This means that the actual dimensions of the crystal are determined by very weak argentophilic (‘silver-loving’) interactions between the silver atoms. When heated, these ‘bonds’ expand rapidly. The flexible framework hinges like garden lattice fencing to convert this expansion into an equally strong contraction along a perpendicular direction. By combining the low-energy silver-silver interactions with structural flexibility in this way, it becomes possible to design new classes of high-performance materials with unprecedented physical properties.
M Calleja, MJ Conterio, MT Dove, AL Goodwin (University of Cambridge), JSO Evans, L Peters (University of Durham), DA Keen, MG Tucker (ISIS)
Contact: Dr AL Goodwin, email@example.com
Research date: December 2008
AL Goodwin et al., Science 319 (2008) 794.