Metalnonmetal the FeO phase diagram

In addition to the metal-metal phase diagrams discussed to this point, the other large class of phase diagrams are the metal-nonmetal systems. In particular, the most important nonmetal is oxygen. Of all the metal/oxygen systems, the Fe/O phase diagram, shown in Fig. 8.15, has received the closest study. On the diagram, the concen-

Fig. 8.15 The Fe/O phase diagram with oxygen pressure isobars.

tration units are the O/Fe atom ratio rather than the mole fraction. The diagram begins at O/Fe = 1 ends at O/Fe = 1.5. The latter corresponds to Fe2O3, with the highest oxidation state of the metal 3+ (ferric). The other valence of iron is 2+ (ferrous). The diagram does not continue to O/Fe = 0 because there is no change from the two-phase regions listed on the left-hand side. Heavy horizontal lines in the diagram separate two-phase regions. The dashed lines represent isobars of the oxygen pressure in equilibrium with each point in the diagram. Single-phase regions are shaded

Other features of the diagram include:

• The 2+ valence state of iron should result in the compound FeO. However what actually exists is a broad (wide range of O/Fe) single-phase oxide called wustite, which is FeO deficient in Fe2+. This oxide is symbolized by "FeO", even though it is not a stoichiometric FeO (i.e., it does not have an oxygen-to-iron ratio of unity). Alternatively, wustite is written as Fei_xO, x indicating the degree of hyperstoichiometry (meaning O/Fe > 1) To maintain electrical neutrality, some of the iron must be in the 3+ state.

The single congruently-melting (meaning that the solid and the liquid into which it transforms have the same O/Fe ratio) compound magnetite, Fe3O4. Since ions cannot have a fractional valence, this compound is a mixture of 2/3 Fe3+ and 1/3 Fe2+. The average valence is 8/3 which, multiplied by 3, balances the 8 negative charges from the four O2- ions in the compound.

The single-phase magnetite region of the phase diagram consists of the iron-deficient compound Fe3-yO4. This O/Fe ratio requires that the fraction of Fe3+ be greater than the value of 2/3 in the stoichiometric compound Fe3O4.

The highest oxide, hematite, has a very narrow single-phase region above 1200oC. This oxide is oxygen-deficient, with the formula Fe2O3-x.

Two liquids exist above 1535oC: metallic iron and a liquid oxide.

• The diagram shows two eutectics,

- an oxygen-rich one at point A at the confluence of the magnetite and hematite solids and a liquid oxide

- another at point B where the two solids are wustite and y-Fe and the liquid is again an oxide.

• Point C is a eutectic-like invariant point involving a-Fe, magnetite and wustite.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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  • gorbaduc puddifoot
    Is FeO nonmetal oxide?
    8 years ago

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