Link to Densify: Topological Transitions and Origin of Hysteresis During the Compression and Decompression of Amorphous Ices

In this Letter, we study the phase transition between amorphous ices and the nature of the hysteresis cycle separating them. We discover that a topological transition takes place as the system transforms from low-density amorphous ice (LDA) at low pressures to high-density amorphous ice (HDA) at high pressures. Specifically, we uncover that the hydrogen bond network (HBN) displays qualitatively different topologies in the LDA and HDA phases: the former characterized by disentangled loop motifs, with the latter displaying topologically complex long-lived Hopf-linked and knotted configurations. At the phase transition, the transient opening of the HBN topological motifs yields mechanical fragility on the macroscale. Our results provide a detailed microscopic description of the topological nature of the phase transition and the hysteresis cycle between amorphous ices. We argue that the topological transition discovered in this work may not only improve our understanding of amorphous ices, but also represent a generic mechanism for the densification of network-forming materials.