Role of symmetry in the orientationally disordered crystals of hard convex polyhedra

TL;DR

This study computationally investigates how symmetry relationships between polyhedral particles and crystal structures influence the formation of a discrete plastic crystal phase at high packing fractions, revealing symmetry as a key controlling factor.

Contribution

It uncovers the direct connection between particle and crystal symmetry point groups and their role in stabilizing the discrete plastic crystal phase at higher densities.

Findings

Symmetry relationships control the existence of the discrete plastic crystal phase.

Point group alignment correlates with the presence of discrete orientations.

Symmetry-based signatures can predict orientational correlations in disordered crystals.

Abstract

The crystalline solids with lack of orientational ordering of anisotropic particles serve the purpose of studying the disordered systems with many fundamental applications in contemporary research. Despite the orientational disorder, multiple unique orientations with fixed angular differences exist in the crystal structures giving rise of "discrete plastic crystal" phase where the particles jump discretely within the unique orientations. We report the computational evidence of the role of symmetries between polyhedral particles and respective crystalline structures in controlling the existence of such phase at comparatively higher range of packing fractions beyond the freely rotating plastic crystals. The point groups of the particle and crystal structure were found to be directly connected in terms of the parallel alignment between the highest order rotational symmetry axes of the particle point group and any rotational axes of crystallographic point group, as a characteristic feature of this phase giving rise of discrete orientations. Based on our previous research [Kundu \textit{et al.}, arXiv:2311.06799, 2023] and new findings reported here, this symmetry relationship appeared to occur at the unit cells of the crystal structures which acted as the source of correlation, where as, all previously reported conserved orientational attributes i.e., number of unique orientations with fixed angular differences, equal population densities within the unique orientations, could be thought as the signatures of correlation present in the entire system. This relationship appeared to control all the aspects of phase which might be useful to draw fundamental insights about the disordered phases with orientational correlation as well as designing the disorder in the crystals.