ISOMERIZATION AND PHASE-TRANSITIONS IN SMALL SODIUM CLUSTERS

Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na4, Na8, and Na-20). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na4 and Na8, and two phase transitions for Na-20. As concerns Na4 and Na8, the transition occurs at almost-equal-to 200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na-20, the two observed phase transitions can be described as a melting of the surface atoms (at almost-equal-to 200 K) preliminarily to the fluctuation of an inner icosahedron seed (at almost-equal-to 300 K).