We report on the discovery and characterization of ice β-XV, which represents the second hydrogen ordered polymorph related to ice VI.

We present the discovery of an unusually large isotope effect in the structural relaxation and the glass transition temperature T g of water. Dielectric relaxation spectroscopy of low-density as well as of vapor-deposited amorphous water reveal T g differences of 10 ± 2 K between H 2 O and D 2 O, sharply contrasting with other hydrogen-bonded liquids for which H/D exchange increases T g by typically less than 1 K. We show that the large isotope effect and the unusual variation of relaxation times in water at low temperatures can be explained in terms of quantum effects. Thus, our findings shed new light on water's peculiar low-temperature dynamics and the possible role of quantum effects in its structural relaxation, and possibly in dynamics of other low-molecularweight liquids.dynamics of water | isotope effect | quantum effects | glass transition | amorphous ice A lthough water is arguably the most important liquid for life, many of its properties remain puzzling (1, 2). In particular, its behavior in the "no-man's land" between 240 K and 150 K, its lowtemperature structural relaxation, and even its glass transition temperature (T g ) continue to be topics of active discussion (3-7).The unusually weak temperature dependence of viscosity near T g ∼136 K, estimated indirectly from crystallization rates, has long been known as one of water's startling features (8). In glassforming liquids the temperature dependence of viscosity or structural relaxation time τ is usually characterized by the fragility index (9):Materials such as SiO 2 and BeF 2 display Arrhenius-like τ(T) behavior with m ∼20-22 and are called strong, whereas those with fragility indices m ∼80 and higher exhibit pronounced superArrhenius variations of τ and are called fragile. Recent dielectric studies discovered an extremely weak temperature dependence of τ in low-density amorphous (LDA) water, with m ∼14 (10). This is by far the lowest fragility known for any liquid and even below its accepted lower limit, m ∼16 (9). Recent speculations ascribe this "superstrong" behavior of water to the impact that zero-point quantum fluctuations can have on structural dynamics (7). Because of the eminent role the atomic mass plays for quantum effects, H/D isotope substitution should have a significant bearing on the low-temperature dynamics of water. To address this question, we performed dielectric measurements on H 2 O and D 2 O prepared as LDA and vapor-deposited water (amorphous solid water, ASW). Details regarding the preparation of LDA water were presented earlier (10) and are briefly summarized in Materials and Methods together with ASW preparation, measurements details, and data analysis (for more details, see also Supporting Information). All of the measurements were repeated several times to confirm data reproducibility. The relaxation times τ for protonated LDA (H-LDA) water (Fig. 1A) as well as for ASW (Fig. 1B), although differing for the reasons given in Supporting Information, both confirm their extremely low fragility, m ∼14 ± 1. An Arrhenius ap...

The pressure–temperature phase diagram of ice displays a perplexing variety of structurally distinct phases. In the century-long history of scientific research on ice, the proton-ordered ice phases numbered XIII through XV were discovered only recently. Despite considerable effort, none of the transitions leading from the low-temperature ordered ices VIII, IX, XI, XIII, XIV and XV to their high-temperature disordered counterparts were experimentally found to display the full Pauling entropy. Here we report calorimetric measurements on suitably high-pressure-treated, hydrogen chloride-doped ice XIV that demonstrate just this at the transition to ice XII. Dielectric spectroscopy on undoped and on variously doped ice XII crystals reveals that addition of hydrogen chloride, the agent triggering complete proton order in ice XIV, enhances the precursor dynamics strongest. These discoveries provide new insights into the puzzling observation that different dopants trigger the formation of different proton-ordered ice phases.

Dielectric spectroscopy measurements are carried out in the temperature range from about 100 to 145 K on nominally pure ice V as well as on crystals doped with KOH and with HCl in order to investigate their reorientation dynamics at ambient pressure. The orientational glass transition temperature of pure ice V is detected at 123 K, in agreement with previous indications from calorimetry. KOH doped ice V displays an about 60-fold enhanced hydrogen dynamics and the dipolar relaxation induced by HCl doping is even by a factor of about 40 000 faster than that of the undoped material. The phase transition of HCl doped ice V to ice XIII is accompanied by a significant reorientational slowdown and a pronounced freeze-out of the electrical susceptibility. The results obtained near this transition are discussed in relation to other order/disorder ice pairs such as ice I/XI and ice XII/XIV.

Above its glass transition, the equilibrated high-density amorphous ice (HDA) transforms to the low-density pendant (LDA). The temperature dependence of the transformation is monitored at ambient pressure using dielectric spectroscopy and at elevated pressures using dilatometry. It is found that near the glass transition temperature of deuterated samples, the transformation kinetics is 300 times slower than the structural relaxation, while for protonated samples, the time scale separation is at least 30 000 and insensitive to doping. The kinetics of the HDA to LDA transformation lacks a proton/deuteron isotope effect, revealing that this process is dominated by the restructuring of the oxygen network. The x-ray diffraction experiments performed on samples at intermediate transition stages reflect a linear combination of the LDA and HDA patterns implying a macroscopic phase separation, instead of a local intermixing of the two amorphous states.