Packing and Self-assembly of Truncated Triangular Bipyramids

TL;DR

This study explores the packing and self-assembly behaviors of truncated triangular bipyramids, identifying dense packings, their dependence on truncation, and which structures form during self-assembly simulations.

Contribution

It provides the first detailed analysis of dense packings and self-assembly of truncated triangular bipyramids, revealing how truncation affects packing density and assembly outcomes.

Findings

Eight distinct dense packings identified with varying truncation.

Only one of the eight packings tends to self-assemble in simulations.

A previously reported quasicrystal structure also forms at high truncation levels.

Abstract

Motivated by breakthroughs in the synthesis of faceted nano- and colloidal particles, as well as theoretical and computational studies of their packings, we investigate a family of truncated triangular bipyramids. We report dense periodic packings with small unit cells that were obtained via numerical and analytical optimization. The maximal packing fraction $\phi_{\max}$ changes continuously with the truncation parameter $t$. Eight distinct packings are identified based on discontinuities in the first and second derivatives of $\phi_{\max}(t)$. These packings differ in the number of particles in the fundamental domain (unit cell) and the type of contacts between the particles. In particular, we report two packings with four particles in the unit cell for which both $\phi_{\max}(t)$ and $\phi'_{\max}(t)$ are continuous and the discontinuity occurs in the second derivative only. In the self-assembly simulations that we perform for larger boxes with 2048 particles, only one out of eight packings is found to assemble. In addition, the degenerate quasicrystal reported previously for triangular bipyramids without truncation [{http://prl.aps.org/abstract/PRL/v107/i21/e215702}{Haji-Akbari \emph{et al.}, \emph{Phys. Rev. Lett.} \textbf{107}: 215702 (2011)}] assembles for truncations as high as 0.45. The self-assembly propensities for the structures formed in the thermodynamic limit are explained using the isoperimetric quotient of the particles and the coordination number in the disordered fluid and in the assembled structure.