BEHAVIOR OF POINT-DEFECTS IN A MODEL CRYSTAL NEAR MELTING

Premelting near point defects has on many occasions been proposed as a melting mechanism, most recently to interpret x-ray-absorption fine-structure and Mossbauer results for impurities in a lead crystal. We studied the behavior of point defects in a model soft-core crystal. Because of its small density change upon melting, the soft-core crystal should be better than hard-core models in accommodating a liquid droplet that has a larger specific volume than the solid. We have failed to observe premelting near isolated point defects (vacancies, polyvacancies, and impurities) in an inverse-6 soft-core model. When vacancies and polyvacancies were deliberately surrounded by a liquid like region in numerical simulations, this region spontaneously ordered under all conditions where the solid was thermodynamically stable. This eliminates the possibility that a kinetic barrier prevented the observation of premelting. When impurities of sufficiently great size mismatch from the bulk particles are introduced into the model, the impurities diffuse as interstitials near the melting point, but isolated impurities do not produce unusually disordered regions near melting.