Orientational phasetransition between hexagonal solids in planar systems of hardcyclic pentamers and heptamers

Abstract

Two-dimensional, hard-body systems of cyclic multimers(pentamers and heptamers) are studied by Monte Carlosimulations. Elastic properties (the elastic constants and thePoisson ratio) are computed for hexagonal solid phases withmolecular rotation by using an algorithm based on thestrain-fluctuation method. In both systems a minimum of thePoisson ratio is observed at the same density at which aqualitative change of the orientational singlet distributionfunction occurs. This result confirms that, in each of thestudied systems, a phase transition occurs between twoelastically isotropic solid phases exhibiting orientationaldisorder: a higher-density phase with (strongly) hinderedrotation and a lower-density phase with (almost) free rotation.The study also suggests that it is worth paying moreattention to the dependence of the Poisson ratio on thethermodynamic variables (volume, pressure or temperature) insolids because this quantity can play the role of a sensitiveindicator of (at least some) structural phase transitions.