Observation of the 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 $970\pm70$ K, the system undergoes a phase transition from a polyhedral form (sharp edges and sharp vertex) to a rounded form (sharp edges, rounded vertex). 
Andrzej Szczepkowicz and Robert Bryl