Investigation of the properties and phase behavior of noncrystalline water is hampered by rapid crystallization in the so-called "no man's land." We here show that it is possible to shrink the no man's land by lifting its low-temperature boundary, i.e., the pressure-dependent crystallization temperature T x (p). In particular, we investigate two types of high-density amorphous ice (HDA) in the pressure range of 0.10-0.50 GPa and show that the commonly studied unannealed state, uHDA, is up to 11 K less stable against crystallization than a pressure-annealed state called eHDA. We interpret this finding based on our previously established microscopic picture of uHDA and eHDA, respectively [M. Seidl et al., Phys. Rev. B 88, 174105 (2013)]. In this picture the glassy uHDA matrix contains ice I h -like nanocrystals, which simply grow upon heating uHDA at pressures 0.20 GPa. By contrast, they experience a polymorphic phase transition followed by subsequent crystal growth at higher pressures. In comparison, upon heating purely glassy eHDA, ice nuclei of a critical size have to form in the first step of crystallization, resulting in a lifted T x (p). Accordingly, utilizing eHDA enables the study of amorphous ice at significantly higher temperatures at which we regard it to be in the ultraviscous liquid state. This will boost experiments aiming at investigating the proposed liquid-liquid phase transition.

We investigated moderate-temperature oxygen diffusion mechanisms in Sr 2 ScGaO 5 with Brownmillerite structure type. From oxygen isotope 18 O− 16 O exchange experiments we determined that oxygen mobility sets in above 550 °C. Temperature-dependent neutron and X-ray (synchrotron) diffraction experiments allowed us to correlate the oxygen mobility with a subtle phase transition of the orthorhombic room-temperature structure with I2mb space group toward Imma, going along with a disorder of the (GaO 4 ) ∞ -tetrahedral chains. From lattice dynamical simulations we could clearly evidence that dynamic switching of the (GaO 4 ) ∞ -tetrahedral chains from its R to L configuration sets in at 600 °C, thus correlating oxygen diffusion with the dynamic disorder. Oxygen ion diffusion pathways are thus constrained along the onedimensional oxygen vacancy channels, which is a different diffusion mechanism compared to that of the isostructural CaFeO 2.5 , where diffusion of the apical oxygen atoms into the vacant lattice sites are equally involved in the diffusion pathway. The proposed ordered room-temperature structure in I2mb is strongly supported by 17 O, 45 Sc, and 71 Ga NMR measurements, which indicate the presence of crystallographically unique sites and the absence of local disordering effects below the phase transition. The electric field gradient tensor components measured at the nuclear sites are found to be in excellent agreement with calculated values using the WIEN2k program. The oxygen site assignment has been independently confirmed by 17 O{ 45 Sc} transfer of adiabatic populations double-resonance experiments.

The crystallisation behaviour of very high-density amorphous ice (VHDA) and unannealed highdensity amorphous ice (uHDA) has been studied in situ by volumetry and ex situ by powder x-ray diffraction in the intermediate pressure range 0.7-1.8 GPa employing different heating rates (0.5, 5 and 30 K min −1 ). This study shows that at pressures >1 GPa the crystallisation behaviour of VHDA and uHDA is basically the same for all heating rates. That is, parallel crystallisation is almost entirely suppressed with mainly ice XII forming. This contrasts former results reporting parallel crystallisation to approximately levelled phase mixtures of ice IV and ice XII even at higher pressures for uHDA. We speculate this to be due to formation of microcracks upon decompression in earlier works, but not in the present one. Crystallisation temperatures T x are up to 16 K higher than previously reported, raising the low-temperature border to no man's land and opening a considerably larger window for future studies on non-crystalline water. The results indicate uHDA to contain heterogeneities on the nanoscale, but VHDA to be rather homogeneous with nano-crystallites being largely absent. Upon transforming uHDA to VHDA, the nano-scale heterogeneities disappear for >1 GPa whereas microcracks do not.Water is a fascinating substance and in many ways its behaviour eludes scientific expectations. These aberrations from the 'norm' have been labelled the anomalies of water and have been described many times in literature 1-3 . One of its puzzling qualities is the great variety of solid phases it can form. Besides the numerous solid crystalline types (polymorphism) 4, 5 also different solid amorphous phases have been identified (polyamorphism) 6,7 . Figure 1 shows the phase-diagram of the thermodynamically stable phases and additionally includes the pressure-temperature regions, in which amorphous ices have been identified. By contrast to the other phases shown in the phase-diagram the amorphous ices are metastable, i.e., there is a phase of crystalline ice lower in Gibbs free energy. In this phase diagram three pressure regimes can be broadly defined. The lower pressure regime can be related to the area of stability of hexagonal ice I h and at lower temperatures of ice XI, i.e., ~0-0.2 GPa. Intermediate pressures 8 (also referred to as "medium pressures" in the literature 9 ) range from ~0.2-2 GPa. This is the richest pressure range, in which water exhibits a broad variety of stable and metastable crystalline phases (ices II-VI, ice IX and ices XII-XV). The higher pressure range is accordingly located at ~p > 2 GPa where the "symmetric" ices VII, VIII and X can be prepared 8 . The amorphous ices have been labelled according to their densities and may occur below T ≈ 180 K and p < 3.5 GPa. Amorphous ice of low density can be obtained by water vapour deposition on a cold substrate 10 , hyperquenching of micrometer-sized droplets onto a cryoplate 11 or by decompression of high-density amorphous ice (HDA) at elevated temperatures 12 . The low-d...

Several proton-disordered crystalline ice structures are known to proton order at sufficiently low temperatures, provided that the right preparation procedure is used. For cubic ice, ice Ic, however, no proton ordering has been observed so far. Here, we subject ice Ic to an experimental protocol similar to that used to proton order hexagonal ice. In situ FT-IR spectroscopy carried out during this procedure reveals that the librational band of the spectrum narrows and acquires a structure that is observed neither in proton-disordered ice Ic nor in ice XI, the proton-ordered variant of hexagonal ice. On the basis of vibrational spectra computed for ice Ic and four of its proton-ordered variants using classical molecular dynamics and ab initio simulations, we conclude that the features of our experimental spectra are due to partial proton ordering, providing the first evidence of proton ordering in cubic ice. We further find that the proton-ordered structure with the lowest energy is ferroelectric, while the structure with the second lowest energy is weakly ferroelectric. Both structures fit the experimental spectral similarly well such that no unique assignment of proton order is possible based on our results.