What is medium-density amorphous ice?*

Polyamorphism in water, characterized by the coexistence of two primary amorphous forms, low-density amorphous (LDA) and high-density amorphous (HDA), each associated with distinct liquid states, low-density liquid (LDL) and high-density liquid (HDL), has long been an intriguing phenomenon. Recent research has unveiled a new dimension of complexity with the discovery of medium density amorphous (MDA) ice, raising questions about its relationship with LDA, HDA, and its connection to the liquid state (Rosu-Finsen et al., Science 2023, 379, 474-478). In this study, we employ molecular dynamics simulations to investigate the formation of MDA. Our analysis reveals that MDA exhibits properties that bridge the characteristics of both the liquid phase and HDA, yet it possesses distinct attributes and a unique formation mechanism. These observations firmly establish MDA as another distinct phase of water.