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 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 thermodynamics. 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 formed at constant shear rate is a steady state non-equilibrium shear-driven amorphous ice (SDA), that can be produced by shearing ice Ih, LDA, or HDA. Our results suggest that MDA could be obtained by ball-milling water glasses without crystallization interference. Increasing the shear rate at ambient pressure produces SDAs with densities ranging from LDA to HDA, revealing shear rate as a new thermodynamic variable in the non-equilibrium phase diagram of water. Indeed, shearing provides access to amorphous states inaccessible by controlling pressure and temperature alone. SDAs produced with shearing rates as high as 10^6 s⁻¹ sample the same region of the potential energy landscape than hyperquenched glasses with identical density, pressure and temperature. Intriguingly, SDAs obtained by shearing at ~10^8 s⁻¹ have density, enthalpy, and structure indistinguishable from those of water “instantaneously” quenched from room temperature to 77 K over 10 ps, making them good approximants for the “true glass” of ambient liquid water.