Iron represents perhaps the best-known example for
allotropy in a
metal. At
atmospheric pressure, there are three allotropic forms of iron:
alpha iron (α) a.k.a.
ferrite, gamma iron (γ) a.k.a.
austenite, and delta iron (δ). At very high pressure, a fourth form exists, called epsilon iron (ε)
hexaferrum. Some controversial experimental evidence exists for another high-pressure form that is stable at very high pressures and temperatures.