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Double Spiral Steps on Ih Ice Crystal Surfaces Grown from Water Vapor Just below the Melting Point

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posted on 07.05.2014, 00:00 by Gen Sazaki, Harutoshi Asakawa, Ken Nagashima, Shunichi Nakatsubo, Yoshinori Furukawa
During the growth of ice crystals, spiral growth is one of the important growth mechanisms that dominate their growth. However, the structure of spiral steps on ice crystal surfaces at temperatures near the melting point has not been identified. Here we show a possible model structure of spiral steps on basal and prism faces of Ih ice crystals grown from water vapor just below 0 °C. We observed in situ the coalescences of spiral steps and steps of two-dimensional (2D) islands by our advanced optical microscopy, which can visualize individual elementary steps (single-bilayer height) of 2D islands on ice basal faces. We found that both spiral steps and 2D island steps on basal and prism faces show single-bilayer heights, though Burgers’ vectors of screw dislocations on both faces need to have double-bilayer heights because of a crystallographic requirement. These results suggest that at the center of a spiral growth hillock, steps of two adjacent single bilayers coincide and then form a double-bilayer step and that the steps of two single bilayers gradually detach themselves with increasing radii of their spiral steps. We observed the appearance of double spiral steps, which strongly supports our model structure, at −15.0 °C (the lowest temperature of our setup).