Low-Temperature Oxygen Migration and Negative Thermal Expansion in ZrW 2 - x Mo x O 8

The ZrW 2 O 8 family of materials has been shown to display the unusual property of an isotropic bulk contraction in volume as a function of temperature. We report here on negative thermal expansion properties and oxygen migration at very low temperatures in ZrW 2 - x Mo x O 8 phases. ZrWMoO 8 shows significant negative thermal expansion (α l = −8.7 × 10 - 6 K - 1 ) and oxygen migration at temperatures as low as 200 K with an activation energy of 0.23 eV. We are unaware of oxygen migration at such low temperatures in other electronic insulators. The low-temperature migration is associated with an order−disorder transition between the α and β forms of the material with Δ V disorder = −0.24%. A negative volume change for an order−disorder transition is unusual.