Negative Thermal Expansion Materials

The ZrW2O8 family of materials has been shown to exhibit the unusual property of negative thermal expansion over a wide temperature range. alpha-ZrW2O8 itself shows negative thermal expansion from 2 K to 1443 K with a coefficient of thermal expansion alpha(1) of -9.1 x 10(-6) K-1 between 2 and 300 K. This behaviour can be related to the unusual structure of these materials, which possess a family of low energy phonon modes with negative Gruneisen parameters. alpha-ZrW2O8 also shows a phase transition at 448 K to beta-ZrW2O8 in which oxygen atoms are dynamically disordered. These phenomena have been investigated by a number of techniques. Results of powder neutron diffraction at 260 different temperatures are presented.