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Iron(II) oxide

physical science Maturity 5-7 Next article →

Iron(II) oxide is an inorganic compound with the chemical formula FeO. It is also frequently called ferrous oxide. In its natural mineral form, this substance is known as wüstite. It appears as a black-colored powder. People sometimes mistake it for rust. However, rust is actually a different substance called hydrated iron(III) oxide.

The structure of iron(II) oxide is very organized. It adopts what scientists call a cubic, rock salt structure. In this arrangement, iron atoms are octahedrally coordinated by oxygen atoms. This means each iron atom is surrounded by oxygen atoms in a specific shape. The oxygen atoms are also octahedrally coordinated by iron atoms. This creates a repeating pattern in the solid.

Sometimes, the chemical mix in the compound is not perfectly balanced. This phenomenon is known as non-stoichiometry. In these cases, the compound is iron deficient. The composition can range from Fe0.84O to Fe0.95O. This happens because iron(II) can easily undergo oxidation to become iron(III). When this occurs, a small portion of iron(II) is replaced by iron(III). These new atoms take up tetrahedral positions within the oxide lattice.

Scientists use specific methods to prepare iron(II) oxide in a laboratory. One method involves the thermal decomposition of iron(II) oxalate. This process must be conducted under an inert atmosphere. An inert atmosphere prevents the formation of iron(III) oxide. Another method can create stoichiometric FeO. This requires heating Fe0.95O with metallic iron. This procedure must reach a temperature of 770 °C. It also requires a very high pressure of 36 kbar.

Iron(II) oxide behaves differently depending on its temperature. It is thermodynamically unstable at temperatures below 575 °C. At these lower temperatures, it tends to disproportionate. This means it breaks down into metal and Fe3O4. The physical structure also changes when it is melted. In a molten state, iron atoms are coordinated by mostly 4 or 5 oxygen atoms. If the temperature drops below 200 K, the symmetry changes to rhombohedral. At this stage, the samples become antiferromagnetic.

We can find this compound deep within our planet. Iron(II) oxide makes up approximately 9% of the Earth's mantle. The mantle is the thick layer located beneath the Earth's crust. Scientists believe iron(II) oxide may be electrically conductive within the mantle. This conductivity might explain certain perturbations in Earth's rotation. These rotations are not accounted for by current models of mantle properties.

Because of its properties, iron(II) oxide has many practical uses. It is commonly used as a pigment to provide color. The FDA has approved its use in cosmetics. It is also used as an ingredient in some tattoo inks. Beyond art and beauty, it has a role in science and hobbyist care. It can be used as a phosphate remover for home aquaria. This helps manage the water chemistry in fish tanks.

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