Diverse densest ternary sphere packings

TL;DR

This study computationally explores the densest arrangements of three different sphere sizes, discovering 22 new structures and analyzing their properties, which could serve as prototypes for complex crystal structures.

Contribution

The paper presents an exhaustive computational exploration of densest ternary sphere packings, identifying 22 new putative structures and analyzing their symmetry and packing efficiency.

Findings

Discovered 22 new putative densest ternary sphere packings.

Highest packing fractions often achieved by phase separation of binary packings.

Some structures exhibit high symmetry and ordered arrangements.

Abstract

The exploration of the densest sphere packings is a fundamental problem in mathematics and a wide variety of sciences including materials science. We present our exhaustive computational exploration of the densest ternary sphere packings (DTSPs) for 451 radius ratios and 436 compositions on top of our previous study [Koshoji and Ozaki, Phys. Rev. E 104, 024101 (2021)]. The unbiased exploration discovers diverse 22 putative DTSPs, and thereby 60 putative DTSPs are identified in total including the 38 DTSPs discussed by the previous study. Some of the discovered DTSPs are well-ordered, for example, the medium spheres in the (9-7-3) structure are placed in a straight line with comprising the unit cell, and the DTSP has the $Pm \bar{3}m$ symmetry if the structural distortion is corrected. At a considerable number of radius ratios, the highest packing fractions are achieved by the phase separations consisting of only the FCC and/or the putative densest binary sphere packings (DBSPs) for all compositions, and the tendency is getting evident as the small and medium spheres are getting larger. The result seems to indicate directly that the local structures in the DBSPs may be denser than those consisting of three kinds of spheres. However, the unit cell of undiscovered DTSPs might only be much larger than in this study due to the complexity of the ternary local structures. Finally, we discuss the correspondence of the DTSPs with real crystals based on the space group. Our study suggests that the diverse structures of DTSPs can be effectively used as structural prototypes for searching ternary, quaternary, and quinary crystal structures.