Solid State Phase Transformations

The crystal structure of a metal is not necessarily stable at all temperatures below the melting temperature. This is due to the fact that the solid always assumes the crystal structure with the lowest Gibbs free energy, even if there are other crystal structures with a slightly higher free energy. This holds, in particular, for metals since the binding energy E₀ of a metal depends relatively little on its atomic arrangement. For instance, the change of heat of fusion of sodium from the bcc to the hexagonal structure amounts to only E₀/1000 (at 36 K). The major contributions to bonding are determined by the electronic structure, and small changes can cause an instability of the crystal structure, for instance by internal fields in ferromagnetic materials. The latter is the cause for the ferromagnetic bcc structure of iron (α-Fe) at low temperatures.