Which way to low-density liquid water?

When rapidly cooled, a liquid undergoes dynamic arrest, forming an amorphous solid commonly called glass. Amorphous solids can also be created by different routes, for example by destabilizing the crystal structure at low temperature by applying pressure or intense radiation, or by depositing gas molecules on very cold substrates. Even though the resulting materials encode information about the preparation route in their frozen structure, they are usually considered members of the same family. This is not the case when the liquid is water. Back in 1985, Mishima et al. (1) and Mishima (2) reported that water forms two distinct amorphous ices, differing in their density and in local structure (the relative position and orientation of nearby molecules in the glass). In low-density glass [with a density of 0.94 g·cm−3 (3), not very different from that of crystalline ice] each molecule is surrounded by, and hydrogen-bonded to, four neighbors, a realization of a random tetrahedral network. In high-density glass [with a density at ambient pressure of about 1.15 g·cm−3 (4)] each molecule also has four hydrogen bonds on average, but a fifth molecule has entered the first coordination shell. By measuring the density as a function of pressure, Mishima et al. (1) and Mishima (2) showed that the transition between the two amorphous ices is rather sharp and characterized by hysteresis, features that are typically encountered in first-order thermodynamic transitions. These experiments have been seminal, seeding within the scientific community two related but still controversial ideas: the possible existence of more than one glass, a phenomenon nowadays indicated with the word “polyamorphism” (5) and, perhaps more importantly, the possibility that the two glasses are the out-of-equilibrium manifestation of two distinct liquid phases (6). Years after the original experiments of Mishima et al. (1), Perakis et al. (7) show in PNAS that it is nowadays possible—exploiting state-of-the-art X-ray spectroscopy—to observe step-by-step the transformation process leading from properly annealed highdensity amorphous ice [eHDA (8)] to low-density amorphous ice (LDA), this time at ambient pressure on increasing temperature. With the simultaneous measurement of the evolution of the sample structure (via wide-angle X-ray scattering) and sample dynamics (via small-angle X-photon correlation spectroscopy, XPCS) it becomes possible to reveal the real nature of the transition and attempt to answer a long list of unsettled questions. Are we really entitled to speak about polyamorphism in out-of-equilibrium systems? Are the two glasses really different phases (albeit dynamically arrested)? Is the observed steep change of the density first reported by Mishima a