Ice Ic without stacking disorder by evacuating hydrogen from hydrogen hydrate

Abstract

Water freezes below 0 °C at ambient pressure, ordinarily to ice Ih with an ABAB… hexagonal stacking sequence. However, it is also known to produce "ice Ic" nominally with an ABCABC… cubic stacking sequence under certain conditions 1 , and its existence in Earth's atmosphere 2-4 , or in comets 5,6 is debated. "Ice Ic", or called as cubic ice, was first identified in 1943 by König 7 , who used electron microscopy to study the condensation of ice from water vapor to a cold substrate. Subsequently, many different routes to "ice Ic" have been established, such as the dissociation of gas hydrates, warming amorphous ices or annealing high-pressure ices recovered at ambient pressure, freezing of μ-or nano-confined water (see refs in 1 ). Despite the numerous studies on "ice Ic", its structure has not been fully verified, because the diffraction patterns of "ice Ic" show signatures of stackingdisorder 1,8 , and ideal ice Ic without stacking-disorder had not been formed until very recently 9 . 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 that is isostructural with ice Ic 10 . Surprisingly, the stacking-disorder free ice Ic is formed from C2 via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations that occur in the cases of recently discovered ice XVI 11 from neon hydrate, or ice XVII 12 from hydrogen hydrate. The obtained ice Ic shows remarkable thermal stability until the phase transition to ice Ih at 250 K; this thermal stability originates from the lack of dislocations, which promote changes in the stacking sequence 13 . This discovery of ideal ice Ic will promote understanding of the role of stacking-disorder 14 on the physical properties of ice as a counter end-member of ice Ih. * D1 belongs to a water molecule in the host structure. ** D2 belongs to the guest deuterium molecule. Uiso(D2) is constrained to be the same value as Uiso(D1), because of the severe correlation between atomic coordinates and occupancies.

Development of spin-contrast-variation neutron powder diffractometry for extracting the structure factor of hydrogen atoms

“…the changing shape and shifting of diffraction peaks due to P H . This technique will be useful for elucidating hydrogencontaining crystals, such as ice systems (Komatsu et al, 2020), polymers (Grishkewich et al, 2017), inorganics (Yashima et al, 2014) and biomacromolecules (Kovalevsky et al, 2008).…”

Identification of synthesisable crystalline phases of water – a prototype for the challenges of computational materials design

“…interactions play important roles, just like in pharmaceuticals [23][24][25][26] and layered materials, such as graphene-hBN superlattices [27]. Properties and thermodynamic stability can be tuned by isotopic substitution [28][29][30], reminiscent of e.g. other molecular crystals [31][32][33].…”

“…The size of this dataset reflects a key challenge of CMD: the number of locally-stable structures inevitably renders the identification of distinct, synthesisable structures a needle-in-a-haystack problem. Setting aside kinetic effects, a rigorous analysis of experimental relevance requires exploring the phase-diagram not just as a function of temperature and pressure, but also electric fields [63], doping [64], isotopic substitution [28][29][30], presence of guest molecules [65][66][67][68], and other thermodynamic constraints. Since this is not computationally viable for large numbers of structures, it becomes crucial to (i) rationalise the space of locally-stable structures, and (ii) distill structures whose favorable energetics and/or particular structural features provide leverage for stabilisation at conditions different from those of the search.. 1 This selection contains duplicates since the databases are not mutually exclusive 2 and without 3-rings, which would normally induce excessive strain in an ice structure 3 In practice, structures with configurational energy exceeding that of an energy-volume convex hull by a multiple of the free energy differences arising from different proton-ordering and NQE of around 10 meV/H 2 O [61] were eliminated.…”

Routes to cubic ice through heterogeneous nucleation

“…The realization that pristine had not been directly observed sparked renewed interest in and attempts to make it without stacking faults (i.e., with 100% “cubicity”). Significantly, in 2020, del Rosso et al ( 14 ) and Komatsu et al ( 15 ) reported the formation of pure in the laboratory ( 16 ). del Rosso et al ( 14 ) made it by heating a powder of O ice XVII to 160 K under vacuum, and Komatsu et al ( 15 ) made it by decompression of hydrogen hydrate at 100 K. There can be little doubt that these studies represent important milestones in the field.…”