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 clu sters (Na4, Na8, and Na-20). We have applied a Monte-Carlo thermodynam ical method which consists in performing canonical samplings for vario us temperatures, these samplings being reexpressed in the microcanonic al ensemble. This method provides thermodynamical values such as the e ntropy and the specific heat. Their behaviour shows one phase transiti on 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 thes e clusters oscillates between numerous shapes. In the case of Na-20, t he two observed phase transitions can be described as a melting of the surface atoms (at almost-equal-to 200 K) preliminarily to the fluctua tion of an inner icosahedron seed (at almost-equal-to 300 K).