Low-temperature oxygen migration and negative thermal expansion in ZrW2-xMoxO8

The ZrW2O8 family of materials has been shown to display the unusual proper ty of an isotropic bulk contraction in volume as a function of temperature. We report here on negative thermal expansion properties and oxygen migrati on at very low temperatures in ZrW2-xMoxO8 phases. ZrWMoO8 shows significan t negative thermal expansion (alpha(1) = -8.7 x 10(-6) K-1) and oxygen migr ation 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 elect ronic insulators. The low-temperature migration is associated with an order -disorder transition between the a and beta forms of the material with Delt a V-disorder = -0.24%. A negative volume change for an order-disorder trans ition is unusual.