Observation of vertex-rounding transition for a crystal in equilibrium: Oxygen-covered tungsten

Equilibrium crystal shape of oxygen-covered tungsten is followed as a function of temperature using field ion microscopy. In the vicinity of the (111) region, at the temperature $970\pm 70\mathrm{K}$, the system undergoes a phase transition from a polyhedral form (sharp edges and sharp vertex) to a rounded form (sharp edges, rounded vertex).

Figure 1

Thermal evolution of a vertex of equilibrium crystals (Refs. 1, 3). Dark shading denotes the rough, rounded region. In (b), $T_0$ denotes the vertex-rounding temperature, and $T_1$ denotes the edge-rounding temperature.

Figure 2

Thermal evolution of an oxygen-covered tungsten crystal. (a) Microscopic image of the vicinity of the (111) pole. The {211} facets form a pyramid pointing in the [111] direction. (b) A model corresponding to (a). The degree of vertex rounding can be characterized by the distance $d$ between the edge end points. (c) The distance between the edge end points (indicated by arrows) increases with temperature.

Figure 3

The size of the rough region at the (111) crystal pole, characterized by the distance $d$ between the edge end points, increases with temperature.