Wüstite (FeO) constitutes one end member of magnesiowüstite, the fundamental phase in the Earth's lower mantle. In the 1970s Ringwood proposed a model for the Earth's core with wüstite being the major light component. Accordingly, wüstite is presumably the only component occuring in both, the core and in the mantle. Studies of its phase tranformations, redox equilibria etc. are fundamental for understanding the core formation and its interaction with the mantle.
Although the existence field of wüstite is extraordinarily wide, it does not include stoichiometric wüstite, "FeO". The (non)stoichiometry of wüstite is more correctly expressed by the formula Fe _x O with x = 0.83...0.96 being dependent on the oxygen fugacity. Moreover, wüstite is not stable at room temperature. It rather decomposes in a eutectoid reaction to magnetite Fe₃O₄ and metallic iron Fe around 560°C.
In this work, the growth of wüstite single crystals by the micro-pulling-down technique is described. Crystals grown at different pulling rates are characterized by X-ray and microscope techniques. The room temperature lattice constant determined by X-ray diffraction at single crystal accounts for 4.312 Å. This value corresponds to a composition Fe_0.956O near the solubility limit on the iron rich side of the existence field.

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