Negative thermal expansion (NTE), a property whereby a material contracts when it is heated, is important for varied applications from ceramic cooker hobs to high precision optical components. Zirconium tungstate (ZrW2O8) has one of the largest NTEs known over a wide temperature range. Its structure consists of a low-density, flexibly-hinged network of ZrO6 octahedra and WO4 tetrahedra. These structural units are able to rotate and translate more vigorously as the material is heated and this causes a net contraction of the crystal lattice. The mechanism for this is most clearly seen in the local changes within the structure – how an individual polyhedron moves with respect to its neighbours. Total neutron scattering (measuring and modelling Bragg and diffuse scattering together) is very sensitive to these changes and results from ZrW2O8 have been interpreted using computational modeling to show (for example) which atoms become closer and which become further apart on heating. Surprisingly, the locally-increased separation of pairs of WO4 tetrahedra is a key structural feature for enabling NTE in ZrW2O8.
MG Tucker, DA Keen (ISIS), MT Dove, AL Goodwin (University of Cambridge), JSO Evans (University of Durham), SA Wells (Arizona State University, USA)
Research date: December 2006
Dr DA Keen, [email@example.com] MG Tucker et al., Phys. Rev. Lett. 95 (2005) 255501