On Understanding Stacking Fault Formation in Ice

Abstract

Despite dedicated efforts aimed at revealing possible molecular structures of the ice defects associated with stacking faults in ice (I), these molecular arrangements have remained a puzzle. Here we demonstrate how the reorganization of water molecules on different faces of ice (I) can facilitate formation of stacking faults within a crystal. We show that a pair of point defects can manifest a particular combination of coupled five- and eight-membered rings (5-8 rings). These structural motifs can facilitate a shift in layers within an ice (I) crystal, thereby inducing stacking faults. Furthermore, the presence of molecular solutes such as methane at the ice interface appears to trigger the formation of coupled 5-8 rings. The observation of such coupled 5-8 ring defects provides insights into the possible molecular mechanisms of stacking fault formation in ice (I) and has implications for ice crystal growth phenomenology and the consequent physical and chemical properties of ice.

Structure of ice crystallized from supercooled water

“…In general, the ice structure formed in our simulations is always disordered in terms of the stacking sequence, with inevitable fluctuations in Φ c ∶Φ h because of the relatively small number of molecules in the simulation box. Other numerical studies of growth on an ice surface (29,30) and nucleation in the subsurface region (31) also confirm the propensity of ice to grow in a stacking-disordered manner. The presented simulation results are consistent with our analysis of the experimental data The DIFFaX prediction for stacking probabilities Φ c ∶Φ h ¼ 45%∶55% and 55%∶45%, which are less consistent with the data.…”

Extent and relevance of stacking disorder in “ice Ic”

“…The following findings are particularly relevant: (i) Our SEM results show that the morphology of crystallites of stacking-disordered ice is often trigonal or pseudohexagonal. Such trigonal crystals have been observed in cirriform clouds (63); triangular growth morphologies were also found in molecular simulations (62). Clearly, a pseudohexagonal crystal shape cannot be taken as evidence for ice I h (23).…”

“…Numerous references are made to "ice I c " in atmospheric science, in particular in the context of cirrus clouds (20,31,33,56,57). The possible presence of "ice I c " is generally considered in the short life cycles of such clouds and has been studied in some detail in laboratory work (8,20,33,55,58,59) and molecular-scale computer simulations of freezing of supercooled water (8,(60)(61)(62). Although our experimental results for the increasing particle size in the densely packed samples investigated (as a consequence of the intercrystallite mass transfer) cannot be transferred directly to atmospheric conditions, the changes of stacking disorder are intrinsic to the particle and can be expected to take place also in clouds.…”

Communication: Structural interconversions between principal clathrate hydrate structures

“…Similar to the direct structural interconversion between the sH and sII crystals observed here, crossnucleation between crystalline structures that share compatible crystallographic planes has been previously reported, such as for the face-centered cubic and the hexagonal close-packed structures 44 and the cubic and hexagonal ice structures. 22,45 It is well established that gas clathrate hydrates usually exist in the three crystalline phases sI, sII, and sH, and these three phases can coexist and interconvert under appropriate conditions. [9][10][11][12][13][14][15][16] While it was previously unclear how these structures might interconvert, this report has provided molecular simulation evidence for potential structural interconversion pathways between clathrate hydrate structures sI and sH and sII and sH.…”