Abstract
The question of existence and nature of a finitesystem analogue of a second-order phase transition in molecular clusters is discussed. By means of constant-energy molecular dynamics, we have simulated the phase transitions of various free clusters of TeF66 during cooling and heating. Structural changes have been recognized by changes of slope of the caloric curve and by the distributions of the molecular axes of symmetry. The structure seen at about 160K is an orientationally disordered phase. It transforms in two steps to an orientationally ordered structure as cluster cools: 1) an intermediate structure with partial alignment of the molecular axes of symmetry is formed at about 80 K; 2) almost all molecules align their axes of symmetry. We interpret the appearance of the intermediate phase to be a result of an ordering-displacement coupling. In addition, a size-dependent Coriolis force has been identified as another cause generating a metastable phase. As the number N of molecules in a cluster increases, this force decreases as 1/N. We associate the hysteresis of the caloric curve with the finite-size effects and critical slowing down, typical of second order transitions.
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