Melting

Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid.

The Lindemann criterion states that melting occurs because of "vibrational instability", e.g. crystals melt; when the average amplitude of thermal vibrations of atoms is relatively high compared with interatomic distances, e.g. <δu2>1/2 > δLRs, where δu is the atomic displacement, the Lindemann parameter δL ≈ 0.20...0.25 and Rs is one-half of the inter-atomic distance.

However, under carefully created conditions, supercooling, or superheating past the melting or freezing point can occur.

Thermodynamically, the supercooled liquid is in the metastable state with respect to the crystalline phase, and it is likely to crystallize suddenly.

Although Hd and Sd are not true equilibrium thermodynamic parameters and can depend on the cooling rate of a melt, they can be found from available experimental data on viscosity of amorphous materials.

This pre-melting shows its effects in e.g. frost heave, the growth of snowflakes, and, taking grain boundary interfaces into account, maybe even in the movement of glaciers.

It occurs not because of the increase of the atomic kinetic energy, but because of changes of the interatomic potential due to excitation of electrons.