NEGATIVE THERMAL EXPANSION FROM 0.3 TO 1050 K IN ZIRCONIUM TUNGSTATE, ZrW₂O₈. John S.O. Evans, T.A. Mary, A.W. Sleight, Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA.

It is well known that the vast majority of materials expand on heating. Zirconium tungstate, however, shows a negative and essentially linear thermal expansion from 0.3 Kelvin to its decomposition temperature of 1050 Kelvin. The structure remains cubic over the entire temperature range and the contraction is therefore isotropic in nature. Negative thermal expansion in other oxide materials is generally restricted to much narrower temperature ranges and is anisotropic. This material therefore has potential applications in the electronics and ceramic industries as a constituent of composites with overall zero expansion.

The structure has been solved using powder X-ray and neutron diffraction data and refined at 14 temperatures from 0.3 to 693 K. The low temperature structure consists of an open framework of corner sharing ZrO₆ octahedra and WO₄ tetrahedra. Around 430 K the structure undergoes a phase transition from acentric P2₁3 to the centric spacegroup Pa . Above the phase transition the structure can be described in terms of the same ZrO₆ units as at low temperature, but with a disordered arrangement of WO₄ tetrahedra. The nature of this phase transition suggests a high oxygen ion mobility in the structure. The mechanism for negative thermal expansion in this material will be discussed in light of the structural results.