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Structural characterization of ice XIX as the second polymorph related to ice VI.

ABSTRACT: Ice polymorphs usually appear as hydrogen disorder-order pairs. Ice VI has a wide range of thermodynamic stability and exists in the interior of Earth and icy moons. Our previous work suggested ice β-XV as a second polymorph deriving from disordered ice VI, in addition to ice XV. Here we report thermal and structural characterization of the previously inaccessible deuterated polymorph using ex situ calorimetry and high-resolution neutron powder diffraction. Ice β-XV, now called ice XIX, is shown to be partially antiferroelectrically ordered and crystallising in a √2×√2×1 supercell. Our powder data recorded at subambient pressure fit best to the structural model in space group [Formula: see text]. Key to the synthesis of deuterated ice XIX is the use of a DCl-doped D₂O/H₂O mixture, where the small H₂O fraction enhances ice XIX nucleation kinetics. In addition, we observe the transition from ice XIX to its sibling ice XV upon heating, which proceeds via a transition state (ice VI^‡) containing a disordered H-sublattice. To the best of our knowledge this represents the first order-order transition known in ice physics.

SUBMITTER: Gasser TM

PROVIDER: S-EPMC7892819 | biostudies-literature |

REPOSITORIES: biostudies-literature

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

Project description: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.

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Author Correction: Experimental evidence for the existence of a second partially-ordered phase of ice VI.

Project description: Not available

| S-EPMC7960736 | biostudies-literature

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Project description:The crystal structure of the title compound, C(17)H(28)O(5), reported here is a polymorph of the structure first reported by El-Feraly, Al-Yahya, Orabi, McPhail & McPhail [J. Nat. Prod. (1992). 55, 878-883]. It is a derivative of the anti-malaria compound artemisinin and consists primarily of three substituted ring systems fused together. A cyclo-hexane ring (distorted chair conformation) fused to a tetra-hydro-pyran group (distorted chair) is adjacent to an oxacyclo-heptane unit containing an endo-peroxide bridge, giving the mol-ecule its particular three-dimensional arrangement. The crystal packing is stabilized by inter-molecular C-H?O inter-actions between an O atom from the endo-peroxide bridge and H atoms from both the cyclo-hexane and seven-membered oxacyclo-heptane fused rings, as well as between an O atom and H atom from adjacent tetra-hydro-pyran rings. The two polymorphs have the same space group and similar cell parameters for the a and b axes, but significantly different values for the c axis.

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Project description:The title compound, C(15)H(22)N(2)O(3), was obtained as a by-product of oxidative cleavage of 1,3-dicyclo-hexyl-(3-oxo-2,3-dihydro-benzofuran-2-yl)imidazolidine-2,4-dione. Herein, we report the crystal structure of a second polymorph, which was obtained by crystallization from an ethanol solution at 253?K, instead of slow evaporation of the same solvent at room temperature. While the first polymorph [Talhi et al. (2011). Acta Cryst. E67, o3243] crystallized in the non-centrosymmetric space group P2(1)2(1)2(1), this second polymorph crystallizes in the centrosymmetric space group P2(1)/n. Compared to the first polymorph, in the crystal no C=O?C=O inter-actions were found (C?O inter-molecular distance longer than 3.15?Å) and instead, close packing of individual mol-ecular units is mediated by C-H?? and weak C-H?O inter-actions.

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