No abstract

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.

A new phase of ice, named ice XV, has been identified and its structure determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart of ice VI and is thermodynamically stable at temperatures below ∼130 K in the 0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure range of the phase diagram have thus been finally mapped, with only hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is antiferroelectric (P1), in clear disagreement with recent theoretical calculations predicting ferroelectric ordering (Cc).PACS numbers: 64.60. Cn, 61.05.fm, 64.70.kt, 61.50.Ks When water freezes, hydrogen-disordered phases of ice crystallize which exhibit orientational disorder of the hydrogen bonded water molecules [1,2]. Upon isobaric cooling, these phases are expected to undergo transitions to hydrogen-ordered phases in which the water molecules adopt the energetically most favored orientations (cf. Fig. 1). However, because of the highly cooperative nature of molecular reorientation in ice, the sluggish kinetics of molecular reorientation at low temperatures often prevent these ordering transitions occurring, and the formation of 'orientational glasses' is observed instead [3,4].The region in the phase diagram at temperatures below the stability domain of ice VI is still 'uncharted territory' (cf. Fig. 1(a)); it is unknown which phase is thermodynamically stable under these conditions [2]. Ice VI is hydrogen-disordered, and extending its region of stability down to 0 K would result in the problematic situation that a phase, with configurational entropy greater than zero, would be stable at 0 K. The transition of ice VI to a hydrogen-ordered phase upon cooling would be a way out of this dilemma. Alternatively, it could be that the ice II/VI and ice VI/VIII equilibrium lines meet above 0 K at an ice II/VI/VIII triple point below which the ordered ices II and VIII would be the stable phases [2]. Based on unpublished neutron diffraction data, Kamb suggested the formation of a partially hydrogen-ordered ice VI phase [5]. Also, Johari and Whalley reported observing a very slow transformation upon cooling ice VI [6]. However, no diffraction data were obtained in that study. Subsequent neutron diffraction measurements concluded that no significant structural changes were observed when pure ice VI was cooled to low temperatures [7]. In agreement with this conclusion, Raman spectroscopic measurements of ice VI at low temperatures also did not reveal the features expected for hydrogen ordering [8], even when the sample * Electronic address: christoph.salzmann@chem.ox.ac.uk was doped with potassium hydroxide [9], a recipe found to promote hydrogen ordering of ice Ih [10]. Recently, it was shown that doping the hydrogen-disordered ices V and XII with minute amounts of hydrochloric acid helps maintaining dynamic states to low enough temperatures so that the phase transitions to the hydrogen-ordered ices XIII and XIV could be observed [11].The ordering processes in ice lead to eithe...

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.