No abstract

Neutron diffraction with isotope substitution is used to determine the structures of high (HDA) and low (LDA) density amorphous ice. Both "phases" are fully hydrogen bonded, tetrahedral networks, with local order similarities between LDA and ice Ih, and HDA and liquid water. Moving from HDA, through liquid water and LDA to ice Ih, the second shell radial order increases at the expense of spatial order. This is linked to a fifth first neighbor "interstitial" that restricts the orientations of first shell waters. This "lynch pin" molecule which keeps the HDA structure intact has implications for the nature of the HDA-LDA transition that bear on the current metastable water debate.

High-density amorphous ice (HDA), further densi®ed on isobaric heating from 77 K to 165 (177) K at 1.1 (1.9) GPa, relaxes at 77 K and 1 bar to the same structural ``state '' with a density of 1.25 0.01 g cm À3 . Its density is higher by % 9% than that of HDA, and thus it is called very-high-density amorphous ice (VHDA). X-ray diractogram and Raman spectrum of VHDA clearly diers from that of HDA, and the hydrogen-bonded O±O distance increases from 2.82 A Ê in HDA to 2.85 A Ê in VHDA. Implications for the polyamorphism of the amorphous forms of water are discussed.

Two hydrogen ordered phases of ice were prepared by cooling the hydrogen disordered ices V and XII under pressure. Previous attempts to unlock the geometrical frustration in hydrogen-bonded structures have focused on doping with potassium hydroxide and have had success in partially increasing the hydrogen ordering in hexagonal ice I (ice Ih). By doping ices V and XII with hydrochloric acid, we have prepared ice XIII and ice XIV, and we analyzed their structures by powder neutron diffraction. The use of hydrogen chloride to release geometrical frustration opens up the possibility of completing the phase diagram of ice.