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Orientational phase transition between hexagonal solids in planar systems of hard cyclic pentamers and heptamers

Two-dimensional, hard-body systems of cyclic multimers (pentamers and heptamers) are studied by Monte Carlo simulations. Elastic properties (the elastic constants and the Poisson ratio) are computed for hexagonal solid phases with molecular rotation by using an algorithm based on the strain-fluctuation method. In both systems a minimum of the Poisson ratio is observed at the same density at which a qualitative change of the orientational singlet distribution function occurs. This result confirms that, in each of the studied systems, a phase transition occurs between two elastically isotropic solid phases exhibiting orientational disorder: a higher-density phase with (strongly) hindered rotation and a lower-density phase with (almost) free rotation. The study also suggests that it is worth paying more attention to the dependence of the Poisson ratio on the thermodynamic variables (volume, pressure or temperature) in solids because this quantity can play the role of a sensitive indicator of (at least some) structural phase transitions.