Ammonium Fluoride as a Hydrogen-Disordering Agent for Ice

Abstract

The removal of residual hydrogen disorder from various phases of ice with acid or base dopants at low temperatures has been a focus of intense research for many decades. As an antipode to these efforts, we now show using neutron diffraction that ammonium fluoride (NH4F) is a hydrogendisordering agent for the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice VIII with ~31% residual hydrogen disorder illustrating the long-range hydrogen-disordering effect of ND4F.The doped ice VII could be supercooled by ~20 K with respect to the hydrogen-ordering temperature of pure ice VII after which the hydrogen-ordering took place slowly over a ~60 K temperature window. These findings demonstrate that ND4F-doping slows down the hydrogenordering kinetics quite substantially. The partial hydrogen order of the doped sample is consistent with the antiferroelectric ordering of pure ice VIII. Yet, we argue that local ferroelectric domains must exist between ionic point defects of opposite charge. In addition to the long-range effect of NH4F-doping on hydrogen-ordered water structures, the design principle of using topological charges should be applicable to a wide range of other 'ice-rule' systems including spin ices and related polar materials. IntroductionWater's phase diagram has been explored for more than a century leading to the discovery of 17 crystallographically distinct phases of ice so far. [1][2][3][4][5] The water molecules in the various phases of ice are fully hydrogen-bonded which gives rise to 4-fold connected networks following the 'twoin-two-out' ice rules with respect to the donation and acceptance of hydrogen bonds. 6-8 Based on this building principle, a wide range of network topologies can be observed ranging from the high-symmetry ice Ih network with only six-membered rings and the highly complex ice V/XIII network to the two interpenetrating individual networks of ice VII/VIII. [6][7][8] Within the constraints of the ice rules, 9 the water molecules can display orientational disorder and such phases are categorized as hydrogen-disordered. In fact, all phases of ice that can be crystallized from liquid water display hydrogen disorder as can be seen in Figure 1(a). In the case of ices Ih, Isd, IV, VI, VII and XII, full hydrogen disorder is observed in neutron diffraction in the form of half-occupied deuterium sites. [10][11][12][13] Ices III and V on the other hand display partial hydrogen order and some of the fractional occupancies deviate significantly from ½. 13 As required by the third law of thermodynamics, the various hydrogen-disordered phases are expected to transform to their corresponding hydrogen-ordered counterparts upon cooling as long-range orientational order of the water molecules is established. Figure 1(b) shows the crystal structures of the hydrogen-disordered ice VII and its hydrogen-ordered counterpart ice VIII.

Anomalous hydrogen dynamics of the ice VII–VIII transition revealed by high-pressure neutron diffraction

“…It is empirically known that H-ordering occurs at 2 GPa within a fraction of a second and it is therefore believed that ice VII is unquenchable. On the other hand, metastable ice VII can exist in the stable region of ice VIII in cases of low-temperature compression of ice VI (16), or incorporating ionic species into ice VII structure (17)(18)(19). Therefore, if the timescale for H-ordering slows down under pressure and becomes comparable to the time necessary to cool the sample to temperature where the kinetics is frozen in, partially disordered ice VIII should be observable.…”

Observation of the Reversible Ice III to Ice IX Phase Transition by Using Ammonium Fluoride as Anti-Ice II Agent

“…In the case of fully hydrogen-ordered ice VIII, doping with 2.5 mol% ND4F changed the single order parameter from 0 to 0.156. [34] As mentioned above, for ice IX, the changes using the same amount of ND4F doping are 0.034 → 0.137 and 0.051 → 0.169 which correspond to changes of 0.103 and 0.118, respectively. The hydrogen-disordering effect of 2.5 mol% ND4F is therefore slightly less in ice IX than it was for the fully hydrogen-ordered ice VIII.…”

“…However, at least some of this effect is probably due to the ND4F doping which has previously been shown to slow-down the hydrogen ordering kinetics of the ice VII to ice VIII phase transition. [34] Hydrogen-(dis)ordering phase transitions require the collective reorientation of water molecules along travelling defect pathways within the crystal. [25] The incorporation of ND4 + and Fpoint defects is thought to terminate such defect pathways which then overall slows down hydrogen (dis)ordering processes.…”

Ammonium fluoride’s analogy to ice: Possibilities and limitations

“…It is presently unknown how (or if at all) NH 4 F loses its molecular character under compression; its phase diagram has not been studied beyond 30 GPa. Finally, while small amounts of NH 4 F doping into ice can modify water clathrate cage structures, manipulate hydrogen ordering transitions and even influence the high-pressure phase diagram, [24][25][26][27][28] it is not known if pure NH 4 F or NH 4 F-rich solutions can form host-guest compounds similar to gas hydrates.…”