Atomic distribution and local structure in-situ VII from in situ neutron diffraction.

Ice polymorphs show extraordinary structural diversity depending on pressure and temperature. The behavior of hydrogen-bond disorder not only is a key ingredient for their structural diversity but also controls their physical properties. However, it has been a challenge to determine the details of the disordered structure in ice polymorphs under pressure, because of the limited observable reciprocal space and inaccuracies related to high-pressure techniques. Here, we present an elucidation of the disordered structure of ice VII, the dominant high-pressure form of water, at 2.2 GPa and 298 K, from both single-crystal and powder neutron-diffraction techniques. We reveal the three-dimensional atomic distributions from the maximum entropy method and unexpectedly find a ring-like distribution of hydrogen in contrast to the commonly accepted discrete sites. In addition, total scattering analysis at 274 K clarified the difference in the intermolecular structure from ice VIII, the ordered counterpart of ice VII, despite an identical molecular geometry. Our complementary structure analyses robustly demonstrate the unique disordered structure of ice VII. Furthermore, these findings are related to proton dynamics, which drastically vary with pressure, and will contribute to an understanding of the structural origin of anomalous physical properties of ice VII under pressures.

Nucleation and growth of crystalline ices from amorphous ices.

We here review mostly experimental and some computational work devoted to nucleation in amorphous ices. In fact, there are only a handful of studies in which nucleation and growth in amorphous ices are investigated as two separate processes. In most studies, crystallization temperatures Tx or crystallization rates RJG are accessed for the combined process. Our Review deals with different amorphous ices, namely, vapor-deposited amorphous solid water (ASW) encountered in many astrophysical environments; hyperquenched glassy water (HGW) produced from µm-droplets of liquid water; and low density amorphous (LDA), high density amorphous (HDA), and very high density amorphous (VHDA) ices produced via pressure-induced amorphization of ice I or from high-pressure polymorphs. We cover the pressure range of up to about 6 GPa and the temperature range of up to 270 K, where only the presence of salts allows for the observation of amorphous ices at such high temperatures. In the case of ASW, its microporosity and very high internal surface to volume ratio are the key factors determining its crystallization kinetics. For HGW, the role of interfaces between individual glassy droplets is crucial but mostly neglected in nucleation or crystallization studies. In the case of LDA, HDA, and VHDA, parallel crystallization kinetics to different ice phases is observed, where the fraction of crystallized ices is controlled by the heating rate. A key aspect here is that in different experiments, amorphous ices of different "purities" are obtained, where "purity" here means the "absence of crystalline nuclei." For this reason, "preseeded amorphous ice" and "nuclei-free amorphous ice" should be distinguished carefully, which has not been done properly in most studies. This makes a direct comparison of results obtained in different laboratories very hard, and even results obtained in the same laboratory are affected by very small changes in the preparation protocol. In terms of mechanism, the results are consistent with amorphous ices turning into an ultraviscous, deeply supercooled liquid prior to nucleation. However, especially in preseeded amorphous ices, crystallization from the preexisting nuclei takes place simultaneously. To separate the time scales of crystallization from the time scale of structure relaxation cleanly, the goal needs to be to produce amorphous ices free from crystalline ice nuclei. Such ices have only been produced in very few studies.

Thermodynamically Stable Intermediate in the Course of Hydrogen Ordering from Ice V to Ice XIII.

Even though many partially ordered ices are known, it remains elusive to understand and categorize them. In this study, we study the ordering from ice V to XIII using calorimetry at ambient pressure and discover that the transition takes place via an intermediate that is thermodynamically stable at 113-120 K. Our isothermal ordering approach allows us to highlight the distinction of this intermediate from ice V and XIII, where there are clear differences both in terms of enthalpy and ordering kinetics. We suggest that the approach developed in the present work can also reveal the nature of partially ordered forms in the hydrogen order-disorder series of other ice phases.

Hydrogen bond symmetrisation in D2O ice observed by neutron diffraction.

Hydrogen bond symmetrisation is the phenomenon where a hydrogen atom is located at the centre of a hydrogen bond. Theoretical studies predict that hydrogen bonds in ice VII eventually undergo symmetrisation upon increasing pressure, involving nuclear quantum effect with significant isotope effect and drastic changes in the elastic properties through several intermediate states with varying hydrogen distribution. Despite numerous experimental studies conducted, the location of hydrogen and hence the transition pressures reported up to date remain inconsistent. Here we report the atomic distribution of deuterium in D2O ice using neutron diffraction above 100 GPa and observe the transition from a bimodal to a unimodal distribution of deuterium at around 80 GPa. At the transition pressure, a significant narrowing of the peak widths of 110 is also observed, attributed to the structural relaxation by the change of elastic properties.

Crystal structure of potassium chloride monohydrate: water intercalation into the B1 structure of KCl under high pressure. Corrigendum.

In the paper by Yamashita et al. [Acta Cryst. (2022), C78, 749-754], an incorrect phrase is updated.

Enthalpy Change from Pure Cubic Ice Ic to Hexagonal Ice Ih.

The preparation of pure cubic ice without hexagonal stacking faults has been realized only recently by del Rosso et al. ( Nat. Mater. 2020, 19, 663-668) and Komatsu et al. ( Nat. Commun. 2020, 11, 464). With our present calorimetric study on the transition from pure cubic ice to hexagonal ice we are able to clarify the value of the enthalpy change ΔHc→h to be -37.7 ± 2.3 J mol-1. The transition temperature is identified as 226 K, much higher than in previous work on ice Isd. This is due to a catalytic effect of hexagonal faults on the transition, but even more importantly due to a relaxation exotherm that was not properly identified in the past.

The hydrogen-bond network in sodium chloride tridecahydrate: analogy with ice VI.

The structure of a recently found hyperhydrated form of sodium chloride (NaCl·13H2O and NaCl·13D2O) has been determined by in situ single-crystal neutron diffraction at 1.7 GPa and 298 K. It has large hydrogen-bond networks and some water molecules have distorted bonding features such as bifurcated hydrogen bonds and five-coordinated water molecules. The hydrogen-bond network has similarities to ice VI in terms of network topology and disordered hydrogen bonds. Assuming the equivalence of network components connected by pseudo-symmetries, the overall network structure of this hydrate can be expressed by breaking it down into smaller structural units which correspond to the ice VI network structure. This hydrogen-bond network contains orientational disorder of water molecules in contrast to the known salt hydrates. An example is presented here for further insights into a hydrogen-bond network containing ionic species.

Crystal structure of potassium chloride monohydrate: water intercalation into the B1 structure of KCl under high pressure.

A new hydrate form of potassium chloride, KCl·H2O, is identified for the first time by in situ single-crystal X-ray diffraction under high pressure. It has a monoclinic structure with lattice parameters of a = 5.687 (7), b = 6.3969 (3), c = 8.447 (3) Šand ß = 107.08 (8)° at 2.23 (4) GPa and 295 K. The structure of this hydrate has K-Cl alignments similar to the B1 phase of anhydrous KCl, while water molecules intercalate among the ionic species. The coordination structures of the K and Cl atoms can be regarded as the intermediate states between the B1 and B2 phases of KCl. This finding provides a perspective on the structural interpretation of multicomponent materials and an additional candidate for bound water in salt-water systems under high pressure, such as inside of icy bodies.

Ice Ic without stacking disorder by evacuating hydrogen from hydrogen hydrate.

Water freezes below 0 °C at ambient pressure ordinarily to ice Ih, with hexagonal stacking sequence. Under certain conditions, ice with a cubic stacking sequence can also be formed, but ideal ice Ic without stacking-disorder has never been formed until recently. Here we demonstrate a route to obtain ice Ic without stacking-disorder by degassing hydrogen from the high-pressure form of hydrogen hydrate, C2, which has a host framework isostructural with ice Ic. The stacking-disorder free ice Ic is formed from C2 via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations occurring in ice XVI from neon hydrate, or ice XVII from hydrogen hydrate. The obtained ice Ic shows remarkable thermal stability, until the phase transition to ice Ih at 250 K, originating from the lack of dislocations. This discovery of ideal ice Ic will promote understanding of the role of stacking-disorder on the physical properties of ice as a counter end-member of ice Ih.

Crystal structure of a high-pressure phase of magnesium chloride hexahydrate determined by in-situ X-ray and neutron diffraction methods.

A high-pressure phase of magnesium chloride hexahydrate (MgCl2·6H2O-II) and its deuterated counterpart (MgCl2·6D2O-II) have been identified for the first time by in-situ single-crystal X-ray and powder neutron diffraction. The crystal structure was analyzed by the Rietveld method for the neutron diffraction pattern based on the initial structure determined by single-crystal X-ray diffraction. This high-pressure phase has a similar framework to that in the known ambient-pressure phase, but exhibits some structural changes with symmetry reduction caused by a subtle modification in the hydrogen-bond network around the Mg(H2O)6 octahedra. These structural features reflect the strain in the high-pressure phases of MgCl2 hydrates.

Anion photoelectron spectroscopy of free [Au25(SC12H25)18].

Previous theoretical studies have shown that the thiolated gold cluster compound [Au25(SR)18]- can be viewed as a prototypical superatom with a closed electronic structure. The quantized electronic structure of [Au25(SR)18]- has been experimentally demonstrated by optical and electrochemical methods in the dispersed state. Nevertheless, no direct information is available on the energy levels and densities of occupied states. Here, we report the photoelectron spectrum of [Au25(SC12H25)18]- isolated under vacuum for the first time. The spectrum exhibits two distinct peaks, corresponding to electron detachment from the superatomic 1P orbitals and Au 5d orbitals of the Au13 core. The adiabatic electron affinity of [Au25(SC12H25)18]0 was experimentally determined to be 2.2 eV, which is significantly smaller than that of [Au25(SCH3)18]0 predicted theoretically.