Thomas C. Hansen

Extent and relevance of stacking disorder in “ice Ic”

A solid water phase commonly known as “cubic ice” or “ice Ic” is frequently encountered in various transitions between the solid, liquid, and gaseous phases of the water substance. It may form, e.g., by water freezing or vapor deposition in the Earth’s atmosphere or in extraterrestrial environments, and plays a central role in various cryopreservation techniques; its formation is observed over a wide temperature range from about 120 K up to the melting point of ice. There was multiple and compelling evidence in the past that this phase is not truly cubic but composed of disordered cubic and hexagonal stacking sequences. The complexity of the stacking disorder, however, appears to have been largely overlooked in most of the literature. By analyzing neutron diffraction data with our stacking-disorder model, we show that correlations between next-nearest layers are clearly developed, leading to marked deviations from a simple random stacking in almost all investigated cases. We follow the evolution of the stacking disorder as a function of time and temperature at conditions relevant to atmospheric processes; a continuous transformation toward normal hexagonal ice is observed. We establish a quantitative link between the crystallite size established by diffraction and electron microscopic images of the material; the crystallite size evolves from several nanometers into the micrometer range with progressive annealing. The crystallites are isometric with markedly rough surfaces parallel to the stacking direction, which has implications for atmospheric sciences.

Ice perfection and onset of anomalous preservation of gas hydrates

Anomalous preservation is the well-established but little-understood phenomenon of a long-term stability of gas hydrates outside their thermodynamic field of stability. It occurs after some initial decomposition into ice in the temperature range between 240 and 273 K. In situ neutron diffraction experiments reveal that the low-temperature on-set of this effect coincides with the annealing of stacking faults of the ice formed initially. The defective, stacking-faulty ice below 240 K apparently does not present an appreciable diffusion barrier for gas molecules while the annealed ordinary ice Ih above this temperature clearly hinders gas diffusion. This is supported by further experiments showing that the so-called ice Ic formed from various high-pressure phases of ice, gas hydrates or amorphous ices does transform fully to ordinary ice Ih only at temperatures near 240 K, i.e. at distinctly higher temperatures than generally assumed. In this light, some quite disparate observations on the transformation process from ice Ic to ice Ih can now be better understood. The transformation upon heating is a multistep-process and its details depend on the starting material and the sample history. This ‘memory’ is finally lost at approximately 240 K for laboratory time-scale experiments.

Approximations to the full description of stacking disorder in ice I for powder diffraction

Abstract Different descriptions of the stacking disorder of the so-called “cubic” phase “ice Ic” and stacking-faulted hexagonal ice Ih exist. We present an overview of the effect of different stacking disorder interaction ranges s from s = 2 to s = 4 after Jagodzinski on the (neutron) powder diffraction patterns of stacking-disordered ice I, which we propose to name ice Ich. We fit in a systematic approach simulated diffraction data of ice Ich for s up to 4 with a multi-peak approach. In this way we allow for estimating the relative proportion of cubic sequences in the stacking sequences by using readily accessible observables of a diffraction pattern.

Large-moment antiferromagnetic order in overdoped high-Tc superconductor 154SmFeAsO1−xDx

Significance Using neutron powder diffraction measurements on the bulk highest-Tc superconductor 154SmFeAsO1−xDx, we discovered a new antiferromagnetic (AFM) phase in the electron-overdoped regime, x ≥ 0.56 (AFM2). The magnetic moment on Fe in AFM2 reaches 2.73 μB/Fe, which is the largest in all the nondoped iron-based antiferromagnets reported so far. Our theoretical calculations reveal that the AFM2 phase in SmFeAsO1−xHx originates in the kinetic frustration of the Fe-3dxy orbital, in which the Fe-3dxy nearest-neighbor hopping parameter becomes zero. The unique phase diagram, i.e., the highest-Tc superconducting phase adjacent to the strongly electron-correlated phase in heavily electron-doped regime (not nondoped regime), yields important clues to the unconventional origins of superconductivity. In iron-based superconductors, high critical temperature (Tc) superconductivity over 50 K has only been accomplished in electron-doped hREFeAsO (hRE is heavy rare earth (RE) element). Although hREFeAsO has the highest bulk Tc (58 K), progress in understanding its physical properties has been relatively slow due to difficulties in achieving high-concentration electron doping and carrying out neutron experiments. Here, we present a systematic neutron powder diffraction study of 154SmFeAsO1−xDx, and the discovery of a long-range antiferromagnetic ordering with x ≥ 0.56 (AFM2) accompanying a structural transition from tetragonal to orthorhombic. Surprisingly, the Fe magnetic moment in AFM2 reaches a magnitude of 2.73 μB/Fe, which is the largest in all nondoped iron pnictides and chalcogenides. Theoretical calculations suggest that the AFM2 phase originates in kinetic frustration of the Fe-3dxy orbital, in which the nearest-neighbor hopping parameter becomes zero. The unique phase diagram, i.e., highest-Tc superconducting phase adjacent to the strongly correlated phase in electron-overdoped regime, yields important clues to the unconventional origins of superconductivity.

Reply to Bogdan et al.: “Cubic ice” in cirrus clouds under dry and wet conditions

Bogdan et al. (1) discuss the relevance of surface roughness of vapor-deposited so-called “ice Ic” discovered in ref. 2 for ice crystals formed at low temperatures in cirrus clouds. The authors emphasize that for ice nucleation from solute aerosol droplets, the initial ice crystal is coated by residual freeze-concentrated solutions, and they provide images from laboratory freezing experiments on large H2SO4 and NH4-sulfate solution droplets.

Filling Ices with Helium and the Formation of Helium Clathrate Hydrate

We have formed the long-sought He-clathrate. This was achieved by refilling helium into ice XVI, opening a new synthesis route for exotic forms of clathrate hydrates. The process was followed by neutron diffraction; structures and cage fillings were established. The stabilizing attractive van der Waals interactions are enhanced by multiple cage fillings with theoretically up to four helium atoms per large cage and up to one per small cage; He-clathrate hydrates can be considered as a solid-state equivalent of the clustering of small apolar entities dissolved in the liquid state of water. Unlike most other guests, helium easily enters and leaves the water cages at temperatures well below 100 K, hampering applications as a gas storage material. Despite the weak dispersive interactions, the inclusion of helium has a very significant effect on lattice constants; this is also established for helium inclusion in ice Ih and suggests that lattice parameters are arguably the most sensitive measure to gauge dispersive water-gas interactions.