Formation of hexagonal and cubic ice during low-temperature growth

Abstract

From our daily life we are familiar with hexagonal ice, but at very low temperature ice can exist in a different structure--that of cubic ice. Seeking to unravel the enigmatic relationship between these two low-pressure phases, we examined their formation on a Pt (111) substrate at low temperatures with scanning tunneling microscopy and atomic force microscopy. After completion of the onemolecule-thick wetting layer, 3D clusters of hexagonal ice grow via layer nucleation. The coalescence of these clusters creates a rich scenario of domain-boundary and screw-dislocation formation. We discovered that during subsequent growth, domain boundaries are replaced by growth spirals around screw dislocations, and that the nature of these spirals determines whether ice adopts the cubic or the hexagonal structure. Initially, most of these spirals are single, i.e., they host a screw dislocation with a Burgers vector connecting neighboring molecular planes, and produce cubic ice. Films thicker than ∼20 nm, however, are dominated by double spirals. Their abundance is surprising because they require a Burgers vector spanning two molecular-layer spacings, distorting the crystal lattice to a larger extent. We propose that these double spirals grow at the expense of the initially more common single spirals for an energetic reason: they produce hexagonal ice.ice growth mechanisms | molecular surface steps | molecular-layer nucleation | scanning probe microscopy | spiral growth O wing to its ubiquity in nature, ice and its structural properties have inspired widespread interest (1, 2). Not long after the introduction of X-ray diffraction in 1912, the structure of the most common modification of ice, hexagonal ice Ih, had already been investigated extensively (1-6). In 1942, König (7) discovered that at low temperatures water occasionally crystallizes into a different modification, cubic ice Ic. Subsequently, cubic ice has been produced in the laboratory, e.g., by condensing water vapor onto cooled substrates (7-11), by heating amorphous solid water (7-9), by supercooling liquid water droplets (12-14) or clusters (15), or by freezing high-pressure phases of ice and reheating them at atmospheric pressure (16)(17)(18)(19). Cubic ice has been proposed to also occur naturally, e.g., in the earth's atmosphere (13,(20)(21)(22)(23)(24) and in comets (25,26). However, even after thousands of articles dedicated to ice formation have appeared, important questions regarding fundamental growth mechanisms of ice remain. Some of these questions concerning the competition between the two lowpressure crystalline phases ice Ih and ice Ic are addressed in this paper.Ice Ih and ice Ic have been observed to coexist at temperatures up to 240 K (10-12, 18, 27, 28). When ice Ic is heated above 170 K it transforms irreversibly into ice Ih. The release of a measurable amount of heat (on the order of 35 J/mol) (17-19) establishes hexagonal ice as the equilibrium structure above 170 K. Below 170 K no phase transformation has been observed, allowing for the poss...

Anharmonic Nuclear Motion and the Relative Stability of Hexagonal and Cubic ice

“…This lower bound is consistent with, but on the high side of, experimentally measured free energy differences of 0.3 − 1.6 meV/H 2 O [7][8][9][10][11][12][13][14][15]. Notably, the experimental value is rather uncertain, mainly because the free energy difference is very small, and because the Ic samples are typically not fully characterised in terms of stacking faults or proton ordering.…”

Surface premelting of water ice

“…Similar double bilayer structures were observed on the prism and basal plane. Similar features and unambiguous signatures of cubic ice spirals were also identified by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) 116 on an ice sample imaged at 145 K.…”

The vapor pressure over nano-crystalline ice

“…Below about 200 K, however, water may be encountered in the metastable cubic form ice I c (e.g., Hobbs, 1974). Recently, studies using diffraction measurements and numerical simulations showed that samples were not composed of pure cubic ice, but rather exhibited crystalline sequences of cubic ice interlaced with sequences of hexagonal ice (e.g., Hudait et al, 2016;Kuhs et al, 2012;Lupi et al, 2017;Malkin et al, 2015;Shallcross and Carpenter, 1957;Thürmer and Nie, 2013). This ice polymorph has been termed stacking disordered ice I sd .…”