Amorphous-amorphous transitions in granular packings

Polyamorphs are the amorphous counterparts of the distinct crystalline phases that are found in the phase diagram of a substance. For a given system, polyamorphs have different local structures, resulting in distinct macroscopic properties, making them important for fundamental science and applications. While previous studies have mainly focused on polyamorphism occurring in systems with an open network structure, little is known how in systems of non-spherical particles the properties of polyamorphs are affected by the orientational degrees of freedom of the particles. Here we employ computational X-ray tomography to investigate how the structure of granular packs composed of spheres, icosahedra, and dodecahedra depends on their packing density. Characterization of the structure in 3D by means of a novel four-point correlation function demonstrates that these systems show a pronounced growth of the local packing anisotropy when the packing fraction is increased, inducing the emergence of two distinct polyamorphs in systems of non-spherical particles. These two structures differ in the orientational order on intermediate distances, revealing a novel class of polyamorph. The identification of these polyamorphs advances our understanding of the jamming and glass transition and will help the development of new types of functional glasses.