Ice XV: A New Thermodynamically Stable Phase of Ice

Abstract

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 Based on unpublished neutron diffraction data, Kamb suggested the formation of a partially hydrogen-ordered ice VI phase [5].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-Dimensional Infrared Spectroscopy of Isotope-Diluted Low Density Amorphous Ice

“…Among the various forms of ice (at least 15 different crystalline phases are known [1,2]), the amorphous state is of particular interest as it possess properties of both an unordered liquid and an ordered solid state. Three different forms of amorphous ice exist which are distinguished by their densities: low-density (LDA), highdensity (HDA) and very high-density (vHDA) amorphous ices [3,4].…”

Two-dimensional infrared spectroscopy of isotope-diluted ice Ih

“…4,5 It has been proposed that clusters of molecules occur, featuring tetrahedral molecular coordination, [6][7][8][9][10] in ways similar to that of the crystalline phase. In total 15 different crystalline phases of ice have been identified [11][12][13] as well as a variety of amorphous states. 14 One of the intriguing variations among the crystalline forms is that in some cases protons are randomly distributed in an otherwise regular oxygen lattice, following the ice rules.…”

Three-dimensional infrared spectroscopy of isotope-diluted ice Ih

“…Investigations of the vibrational energy transfer have also been extended to the crystalline phase of water with a great emphasis on ice Ih, the naturally occurring phase of the 15 different crystalline ice forms [27] and amorphous states [28]. Ice Ih features hexagonal symmetry with respect to the oxygen lattice (symmetry P 6 3 /mmc), whereas it is disordered with respect to the hydrogen positions (proton disorder) [29].…”