Phase transitions and adjacent phenomena in simple atomic systems

Clusters and bulk systems of bound atoms with pair-wise interactions have two types of excitations: configurational, due to a change in the atomic arrangement in space, and thermal, associated with atomic vibrations. The configurational excitation is responsible for phase transitions in such systems and can be considered as a transition from the global minimum of the atomic potential energy surface in a multidimensional space of atomic coordinates to some other, higher-energy local minima. From this standpoint, various aspects of aggregate states of atomic clusters are considered, including coexistence of the liquid and solid cluster phases, the freezing point as the temperature of transition from the metastable liquid state to the unstable state, the glassy states as unstable configurationally excited states with long lifetimes, and the phase transition under high pressures when the crystal lattice for the distribution of atoms is no longer the most stable form for the solid state. The concept of voids as elementary internal configurational excitations of a macroscopic atomic system, which are connected with local minima of the potential energy surface, allows us to consider the glassy–solid transition and processes of the growth of nuclei of a new phase as a result of void transport. The degrees of deviation from traditional macroscopic thermodynamics for clusters and bulk systems near a phase transition is analyzed. It is shown that the thermal motion of atoms makes a significant contribution to the entropy jump at the phase transition, which allows us to use the Lindemann criterion for the phase transition and other criteria which use parameters of thermal motion of atoms, even though the inherent nature of the phase transition is determined by configurational excitation.