Zirconium Tungstate - Cubic Phase

Cubic Phase

Cubic zirconium tungstate (alpha-ZrW₂O₈), one of the several known phases of zirconium tungstate (ZrW₂O₈) is perhaps one of the most studied materials to exhibit negative thermal expansion. It has been shown to contract continuously over a previously unprecedented temperature range of 0.3 to 1050 K (at higher temperatures the material decomposes). Since the structure is cubic, as described below, the thermal contraction is isotropic - equal in all directions. There is much ongoing research attempting to elucidate why the material exhibits such dramatic negative thermal expansion.

This phase is thermodynamically unstable at room temperature with respect to the binary oxides ZrO₂ and WO₃, but may be synthesised by heating stoichiometric quantities of these oxides together and then quenching the material by rapidly cooling it from approximately 900 °C to room temperature.

The structure of cubic zirconium tungstate consists of corner-sharing ZrO₆ octahedral and WO₄ tetrahedral structural units. Its unusual expansion properties are thought to be due to vibrational modes known as Rigid Unit Modes (RUMs), which involve the coupled rotation of the polyhedral units that make up the structure, and lead to contraction.