Chemistry Reference
In-Depth Information
Figure 5.
Liquid- and solid-phase boundaries of water. Some ice phases appear in pairs (Ih/XI,
VI/XV, VII/VIII) in which the oxygen atoms have nearly the same lattice positions. Hydrogen-bonds
are disordered in the high-temperature member of the pair, and are locked into an ordered arrange-
ment in the low-temperature member. Only equilibrium phases are shown here. Some metastable
phases are depicted in Fig. 6. Several phase boundaries are estimated. The superionic phase [40, 41] is
labeled “SI”. (See insert for color representation of the figure.)
re-forming H-bonds would require passage over very large barriers. Presumably
the introduction of defects provides lower-energy activation pathways for H-bond
rearrangements. Ice XI has been observed to form upon doping of ice Ih with hy-
droxide. Doping with acid promotes the formation of ice XIII, XV, and XIV from
ice V, VI, and XII, respectively [22, 24, 42]. The mechanism by which H-bond
order-disorder transitions are catalyzed by excess protons or hydroxide is poorly
understood. Recent experiments indicate that ionic defects are immobilized on an
accessible experimental time scale somewhere between 100-200 K [43, 44]. If
there are no ionic defects actively diffusing at 72 K, then it is unclear how hydrox-
ide ions catalyze the ice Ih/XI transition. It is also not clear why excess protons
are effective for the ice VI/XV and XII/XIV transitions, while excess hydroxide
catalyzes the ice Ih/XI transition.
The disordered phases of ice discussed above are often regarded as fully disor-
dered phases. However, studies on ices III [45, 46] and V [46], however, have shown
that these phases are only partially disordered (i.e., some H-bond arrangements
