Low-pressure phase diagram of pure iron. BCC is body centered cubic and FCC is face-centered cubic.Iron-carbon eutectic phase diagram, showing various forms of FexCy substances.Iron allotropes, showing the differences in structure. The alpha iron (α-Fe) is a body-centered cubic (BCC) and the gamma iron (γ-Fe) is a face-centered cubic (FCC).
At atmospheric pressure, three allotropic forms of iron exist, depending on temperature: alpha iron (α-Fe, ferrite), gamma iron (γ-Fe, austenite), and delta iron (δ-Fe). At very high pressure, a fourth form exists, epsilon iron (ε-Fe, hexaferrum). Some controversial experimental evidence suggests the existence of a fifth high-pressure form that is stable at very high pressures and temperatures.[1]
The phases of iron at atmospheric pressure are important because of the differences in solubility of carbon, forming different types of steel. The high-pressure phases of iron are important as models for the solid parts of planetary cores. The inner core of the Earth is generally assumed to consist essentially of a crystalline iron-nickel alloy with ε structure.[2][3][4] The outer core surrounding the solid inner core is believed to be composed of liquid iron mixed with nickel and trace amounts of lighter elements.
^Boehler, Reinhard (2000). "High-pressure experiments and the phase diagram of lower mantle and core materials". Reviews of Geophysics. 38 (2). American Geophysical Union: 221–245. Bibcode:2000RvGeo..38..221B. doi:10.1029/1998RG000053. S2CID 33458168.
^Cohen, Ronald E.; Stixrude, Lars. "Crystal at the Center of the Earth". Archived from the original on 5 February 2007. Retrieved 2007-02-05.
^Stixrude, Lars; Cohen, Ronald E. (March 1995). "High-Pressure Elasticity of Iron and Anisotropy of Earth's Inner Core". Science. 267 (5206): 1972–5. Bibcode:1995Sci...267.1972S. doi:10.1126/science.267.5206.1972. PMID 17770110. S2CID 39711239.
^"What is at the centre of the Earth?". BBC News. 31 August 2011.
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