Phase Transition and Effect of Defects on Thermal Expansion in FexO

Introduction Transition-metal monoxides, such as NiO, CoO, FeO, and MnO, have been receiving considerable attention as model materials for elucidating defect structures, the nature of interaction between defects, and effect of nonstoichiometry on physical properties. Wüstite is an iron oxide that possesses rock-salt structure and exhibits a wide range of nonstoichiometry (0.88 < x < 1 in FexO). At atmospheric pressure, the stoichiometry of wüstite is defined by two univariants; one is for wüstite in equilibrium with iron, and the other is for wüstite in equilibrium with magnetite. The eutectoid point is located at 849K and x ≈ 0.945 in the temperature-composition space. The precise values of the eutectoidal temperature and composition at high pressure, however, have not been determined by any previous experiment. While several studies have aimed at determining the wüstite/iron phase boundary, the effect of pressure on the wüstite/magnetite (i.e., oxygen-rich) phase boundary is entirely not known, even though some insights can be inferred from the only available experimental study of Shen et al. [1]. Another important issue deals with the effect of defect concentration on the thermal expansion of wüstite. Overall, only a few attempts have been made to address this issue at atmospheric pressure [2]. This limited effort is primarily due to the fact that such a volume-related thermal property is more difficult to study in wüstite than in most defect-free materials because the arrangement of defect clusters and the extent of magnetite exsolution may adjust to changes in temperature [3, 4]. Thus, an experimentally difficult challenge is to separate the volume changes contributed by metal-oxygen bond expansion from those possibly contributed by continuous stoichiometry variation with temperature. Nevertheless, from available experimental data [2], it is generally concluded that thermal expansion of wüstite does not appear to vary with stoichiometry.