At atmospheric pressure, three allotropic forms of iron exist: alpha iron (α-Fe), gamma iron (γ-Fe), and delta iron (δ-Fe). At very high pressure, a fourth form exists, called epsilon iron (ε-Fe). 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.
https://en.wikipedia.org/wiki/Allotropes_of_iron
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