The (7 × 7) ↔ (1 × 1) phase transition was investigated in situ by low energy electron reflection microscopy (LEERM). Heterogeneous nucleation of the (7 × 7) domains at monoatomic steps is observed as well as homogeneous nucleation in the atomically flat terraces. The density of nuclei and the domain growth velocity increase with the supercooling ΔT of the surface. The domains start to grow at low ΔT in triangular equilibrium shape; interaction of neighbouring domains leads to a dendritic overall shape. The observations strongly support the view that the transition is of first order.
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Telieps et al. (1986) studied this question.
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