High density amorphous at room temperature

Jing-Yin Chen and Choong-Shik Yoo1

Institute for Shock Physics and Department of Chemistry, Washington State University, Pullman, WA 99164–2816

Edited* by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved March 23, 2011 (received for review January 14, 2011)

The phase diagram of is both unusual and complex, exhibit- neighbor oxygen-oxygen distance in ice VII decreases, eventually ing a wide range of polymorphs including proton-ordered or dis- bringing each of the hydrogen atoms to the midway points of ordered forms. In addition, a variety of stable and metastable the neighboring oxygen atoms (i.e., symmetrized ice-X) above – forms are observed. The richness of H2O phases attests the versa- 60 70 GPa (22). Above 150 GPa ice X further transforms to tility of hydrogen-bonded network structures that include kineti- an antifluorite-like structure (23). At higher pressures, it is also cally stable amorphous . Information of the amorphous , predicted to transform to orthorhombic structures such as Pbcm, however, is rarely available especially for the stability field and Pbca, and Cmcm (24). From 40–100 GPa, under increasing tem- transformation dynamics—but all reported to exist below the peratures, ab initio molecular dynamics (MD) simulations have crystallization temperature of approximately 150–170 K below found evidence of enhanced hydrogen self-diffusion and signifi- 4–5 GPa. Here, we present the evidence of high density amorphous cant ionic conductivity below the melting curve of H2O (25, 26). (HDA) ice formed well above the crystallization temperature at Yet, with decreasing temperatures (below 270 K), hydrogen-dis- 1 GPa—well inside the so-called “no-man’s land.” It is formed from ordered ice VII transforms to antiferroelectrically dipole-ordered metastable ice VII in the stability field of ice VI under rapid com- ice VIII (27). Similarly, ice III transforms to dipole-ordered pression using dynamic-diamond anvil cell (d-DAC) and results antiferroelectric ice IX below 170 K (28), and to either from structural similarities between HDA and ice VII. The formation antiferroelectric or ferroelectric ice XI below 70 K (29). follows an interfacial growth mechanism unlike the melting pro- The proton- and dipole-ordering transitions in H2O are gov- cess. Nevertheless, the occurrence of HDA along the extrapolated erned by well beyond the thermodynamic constraints, to strong melt line of ice VII resembles the ice Ih-to-HDA transition, indicating kinetics that gives rise to metastable structures as represented that structural instabilities of parent ice VII and Ih drive the pres- in Fig. 1. Neutron diffraction experiments and classical MD simu- sure-induced amorphization. lations (12, 30) provide evidence for the existence of two different liquids, LDWand HDW. This finding was later explained in terms dynamic-DAC ∣ rapid solidication ∣ high pressure kinetics ∣ metastability of density fluctuation in liquid rather than a first-order phase transition in liquid (31). Under rapid “jet” cooling, water trans- bundant in nature, water is a major constituent of planets forms into LDA, an amorphous , rather than ice Ih at the 106 ∕ T 140 Aand living organisms alike. The phase diagram of water cooling rate of K s and at < K. Under low-tempera- exhibits a large number of polymorphs with great diversity in crys- ture compression (below 150 K) LDA or ice Ih transforms to – talline structure, chemical bonding, and collective interactions HDA, another amorphous solid (4 6). This transition was found (1–3). The hydrogen-bond angles and topology of relatively to occur also to the extrapolated melting line of ice Ih and below weak hydrogen bonds (with respect to cova