Calorimetric Investigation of Hydrogen-Atom Sublattice Transitions in the Ice VI/XV/XIX Trio

Abstract

Ice XIX represents the latest discovery of ice polymorphs and exists in the medium pressure range near 1–2 GPa. Ice XIX is a partially hydrogen-ordered phase, by contrast to its disordered mother phase ice VI, which shares the same oxygen-atom network with ice XIX. Ice XIX differs in terms of the ordering of the hydrogen-atom sublattice, and hence the space group, from its hydrogen-ordered sibling ice XV, which also features the same type of oxygen network. Together, ice VI, XV, and XIX form the only known trio of ice polymorphs, where polymorphic transformations from order to order, order to disorder, and disorder to order are possible, which also compete with each other depending on the thermodynamic path taken and the cooling/heating rates employed. These transitions in the H-sublattice have barely been investigated, so we study here the unique triangular relation in the ice VI/XV/XIX trio based on calorimetry experiments. We reveal the following key features for H-sublattice transitions: (i) upon cooling ice VI, domains of ice XV and XIX develop simultaneously, where pure ice XV forms at ≤0.85 GPa and pure ice XIX forms at ≥1.60 GPa, (ii) ice XIX transforms into ice XV via a transient disordered state, (iii) ice XV recooled at ambient pressure features a complex domain structure, possibly containing an unknown H-ordered polymorph, (iv) recooled ice XV partly transforms back into ice XIX at 1.80 GPa, and (v) partial deuteration slows down domain reordering strongly. These findings not only are of interest in understanding possible hydrogen-ordering and -disordering processes in the interior of icy moons and planets but, more importantly, also provide a challenging benchmark for our understanding and parameterizing many-body interactions in H-bonded networks.

Computer simulation of hypothetical hydrogen ordered structure of ice XIX

“…28 According to Gasser's report, ice XIX is a ''partially hydrogen-ordered'' phase. 4,20 Yamane et al also reported that the unit cell of ice XIX allows many possible hydrogen-ordered configurations. 21 Since the totally hydrogen disordered structure is ice VI, we proposed that the real structure of ice XIX maybe a big supercell cell contains many hydrogen ordered…”

“…3,4 Gasser et al revealed the transformation relationship between ice VI, ice XV and ice XIX based on calorimetry experiments. 20 Yamane et al believed that ice XIX exists at higher pressure, and because the density of ice XIX is greater than that of ices XV and VI, ice XIX is more stable. 21 As mentioned, ice XIX is hydrogen-ordered phase and has the same oxygen sublattice structure as ices VI and XV; the structures of ices XIX and XV differ in the positions of their hydrogen atoms and in their space groups.…”

Accurate crystal structure of ice VI from X-ray diffraction with Hirshfeld atom refinement

“…Additionally, the oxygen topology was found to be the same in ice VI, XV (Salzmann et al, 2009) and XIX (Gasser et al, 2021a(Gasser et al, , 2018Yamane et al, 2021;Salzmann et al, 2021). While disordered phase VI, ordered phase XV and partially ordered phase XIX form a kind of family of ice structures where transitions from ordered to ordered, ordered to disordered, and disordered to ordered are possible (Gasser et al, 2021b). Structural distortions are claimed to be the main structural feature of ice XIX (Salzmann et al, 2021).…”

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

“…These two scenarios need to be distinguished to understand disordered nature as seen in the arguments for ice XIX between the deep-glassy state Rosu-Finsen et al, 2020) and the new hydrogen-ordered phase (Gasser et al, 2018;Thoeny et al, 2019;Gasser, Thoeny, Greussing et al, 2021). Another explanation involving structural distortion was also raised (Salzmann et al, 2021) and the controversy continues.…”