Effects of Defect and Pressure on the Thermal Expansivity in FexO

Pressure-volume-temperature measurements have been carried out using synchrotron X-ray diffraction for wuestite at static pressures of 1.9, 2.6, and 5.4 GPa. Our results revealed that the composition change of wuestite and, hence, rearrangements of defect structures are primarily caused by the magnetite (Fe3O4) exsolution at temperatures of 523-723 K. Based on the isobaric volume-temperature data collected during cooling, the contribution of compositional variations to the unit-cell volumes of wuestite in the ranges of 300-673 K and 723-1073 K is negligibly small, within the experimental uncertainties. These observations suggest that the measured volume changes in the range of 300-673 K and 723-1,073 K can be attributed to the metal-oxygen bond expansion. Owing to the magnetite exsolution, thermal expansion data are obtained in each experiment at 1.9, 2.6, and 5.4 GPa for wuestite of two different compositions, Fe0.987O and Fe0.942O. At all three pressures, Fe0.942O shows a thermal expansion that is about 30% larger than Fe0.987O. Such findings represent the first experimental evidence of a substantial effect of nonstoichiometry on thermal expansivity, and based on previous thermodynamic calculations of the defect formation and interaction, this effect is likely associated with the distinct defects arrangements in iron-rich and more iron-deficient wuestite. This study also presents thermal equations of state for wuestite of two different compositions. Such volume-related properties at high temperatures are experimentally difficult to obtain in wuestite but important for thermodynamic studies in the binary Fe-O system.