Force fields in close-packed crystals and their melts in relation to defects, surface energies and mechanical properties

Abstract

Pair potentials afford a quantitative starting point for studying the vacancy formation energy E _v in hot close-packed crystals such as Ar. A summary will be given of the relation of E _v to the melting temperature T _m, via liquid structure, and a brief comment made on the role of three-body-forces. This leads into a discussion of ‘criteria’ characterizing the solid—liquid phase transition, one of these being the assertion that a close-packed crystal melts when the internal energy required to create a localized hole (the unrelaxed vacancy in the hot crystal) is equal to the change in internal energy at T _m required to expand the liquid by one atomic volume. When N-body forces become important and are treated by so-called glue models, exemplified in the work of R. A. Johnson (1988, Physical Review B, 37, 3924), on a model of Cu metal, the important role of the shear modulus in determining both E _v (now at T = 0) and the divacancy binding energy is stressed. Finally, E _v in, for example, Al is argued to be closely connected with surface energy, through the similarity of conduction-electron density profiles around the vacant site and through the planar surface. This leads to a brief account of unpublished work on mechanical properties in the liquid at T _m, and in particular shear viscosity, which is related to surface tension via the velocity of sound and the thickness of the liquid vapour interface.