Pre-freezing and Pre-melting

When attempts are made to interpret the solid-liquid transition in terms of molecular structure, highly significant information can emerge from phemomena of pre-melting and pre-freezing. Empirically, on approaching the transition between solid and liquid, characteristic parameters of the phase under examination may show an accelerated rate of change, in the sense of a premonition of the change due to take place at the transition point itself. It is convenient to consider separately thermodynamic parameters such as the heat content, the volume, and derived quantities, and rate process parameters which have included critical shear strength and diffusion for the solid, and viscosity and other transport parameters for the melt. In certain cases many other methods have become available in recent years, especially in studying pre-melting. Depending on the nature of the crystal, thermodynamic phenomena of pre-melting may be very prominent, or almost imperceptible. Prominent examples with various crystal structures include heat content and volume changes in polymethylene compounds!' 2, heat content change of lithium fluoride" and alkali metals", and thermal expansions of silver halides", When only thermodynamic measurements are available, rigorous care is always necessary to eliminate a quite trivial effect known as " heterophase pre-melting". This arises from a small mole fraction nlN of impurity when this is not soluble in the crystals but is soluble in the melt. Theory shows that the excess ~Q of volume or heat content change, which arises from actual formation of small amounts of liquid at a temperature T below the limiting melting point T ro, in such cases is related to the total change ~Qo by the equation