Medium-Density Amorphous Ice Unveils Shear Rate as a New Dimension in Water’s Phase Diagram

Recent experiments revealed a new amorphous ice phase, medium-density amorphous ice (MDA), formed by ball-milling ice Ih at 77 K (Rosu-Finsen et al., Science 2023, 379, 474-478). MDA has a density between that of low-density amorphous (LDA) and high-density amorphous (HDA) ice, adding to the complexity of water’s phase diagram, known for its glass polyamorphism and two-state thermody-namics. The nature of MDA and its relation to other amorphous ices and liquid water remain unsolved. Here, we use molecular simula-tions under controlled pressure and shear rate at 77 K to produce and investigate MDA. We find that MDA is not a nanocrystalline state but a shear-driven amorphous ice (SDA). Indeed, MDA formed at a constant shear rate is a non-equilibrium steady state that can be produced by shearing ice Ih, LDA, or HDA. These results suggest that MDA could be obtained by ball-milling water glasses. Increasing the shear rate produces SDAs with densities ranging from LDA to HDA at ambient pressure, revealing shear rate as a new thermodynamic variable in the non-equilibrium phase diagram of water. Shearing provides access to amorphous states that cannot be reached by control-ling pressure and temperature alone. However, SDAs and hyperquenched glasses with identical temperature, pressure, and density have indistinguishable energy and structure, suggesting a common potential energy landscape. Intriguingly, SDAs obtained by shearing at ~10^8 s⁻¹ have density, enthalpy, and structure indistinguishable from those of instantaneously quenched liquid water, indicating that shearing could render the “true glass” of ambient liquid water.