Structural phase transition and orientation-strain glass formation in anisotropic particle systems with impurities in two dimensions

Using a modified Lennard-Jones model for elliptic particles and spherical impurities, we present results of molecular dynamics simulation in two dimensions. In one-component systems of elliptic particles, we find an orientation phase transition on a hexagonal lattice as the temperature $T$ is lowered. It is also a structural one because of spontaneous strain. At low $T$, there arise three martensitic variants due to the underlying lattice, leading to a shape memory effect without dislocation formation. Thermal hysteresis, a minimum of the shear modulus, and a maximum of the specific heat are also found with varying $T$. With increasing the composition $c$ of impurities, the three kinds of orientation domains are finely divided, yielding orientation-strain glass with mesoscopically ordered regions still surviving. If the impurities are large and repulsive, planar anchoring of the elliptic particles occurs around the impurity surfaces. If they are small and attractive, homeotropic anchoring occurs. Clustering of impurities is conspicuous. With increasing the anchoring power and/or the composition of the impurities, positional disorder can also be enhanced. We also investigate the rotational dynamics of the molecular orientations.

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