On the role of intermolecular vibrational motions for ice polymorphs. III. Mode characteristics associated with negative thermal expansion

Coexistence of vitreous and crystalline phases of H2O at ambient temperature

Understanding the phase behavior of H₂O is essential in geoscience, extreme biology, biological imaging, chemistry, and physics. Vitreous phases of H₂O are of particular importance since these phases avoid the typical expansion of H₂O during ice-crystal formation. Here, we confirm the existence of vitreous ice and ice VI in mixed-phase samples of H₂O at room temperature and high pressure. We show how Raman scattering and X-ray diffraction alone lead to misleading characterization and understanding of the mixed-phase material, a conclusion supported by molecular dynamics simulations. The coexistence of vitreous and crystalline components of H₂O under these conditions is crucial for experimental studies of biological systems. The results have implications for related metastable transitions in other materials under pressure.

Formation of vitreous ice during rapid compression of water at room temperature is important for biology and the study of biological systems. Here, we show that Raman spectra of rapidly compressed water at greater than 1 GPa at room temperature exhibits the signature of high-density amorphous ice, whereas the X-ray diffraction (XRD) pattern is dominated by crystalline ice VI. To resolve this apparent contradiction, we used molecular dynamics simulations to calculate full vibrational spectra and diffraction patterns of mixtures of vitreous ice and ice VI, including embedded interfaces between the two phases. We show quantitatively that Raman spectra, which probe the local polarizability with respect to atomic displacements, are dominated by the vitreous phase, whereas a small amount of the crystalline component is readily apparent by XRD. The results of our combined experimental and theoretical studies have implications for detecting vitreous phases of water, survival of biological systems under extreme conditions, and biological imaging. The results provide additional insight into the stable and metastable phases of H₂O as a function of pressure and temperature, as well as of other materials undergoing pressure-induced amorphization and other metastable transitions.