Crystallization of Supercooled Liquid Elements Induced by Superclusters Containing Magic Atom Numbers

A few experiments have already detected the presence of icosahedral superclusters in undercooled liquids, confirming a possible homogeneous nucleation of such entities as suggested by Franck. These superclusters survive in melts above the crystal melting temperature Tm because all their surface atoms have the same fusion heat as their core atoms and are melted by homogeneous nucleation of liquid in their core, depending on overheating time and temperature. In complete contrast to current ideas, a long time is necessary to melt them and to attain the thermodynamic equilibrium above Tm. They act as heterogeneous growth nuclei of crystallized phase at a temperature Tc of the undercooled melt when they are not melted. They contribute to the reduction of the critical barrier, which becomes smaller than that of crystals containing the same atom number n. The undercooling rate is always limited, even in a liquid at thermodynamic equilibrium, because the homogeneous nucleation of 13-atom superclusters reduces the energy barrier, and increases Tc above the homogeneous nucleation temperature equal to Tm/3 in liquid elements. After weak superheating, the most stable superclusters containing n = 13, 55, 147, 309 and 561 atoms survive or melt and determine Tc during undercooling, which depends on n and on the sample volume. The experimental nucleation temperatures Tc of 32 liquid elements and the melting temperatures of superclusters are predicted without any adjustable parameter using a sample volume varying by nearly 18 orders of magnitude. The classical Gibbs free energy change is used, adding an enthalpy saving related to the Laplace pressure change associated with supercluster formation, which is quantified and strongly weakened for n = 13 and 55.