Overlayers on surfaces with square symmetry exhibit a huge variety of strain relief mechanisms. I present a simple 2D Frenkel-Kontorova model and calculate the associated zero temperature phase diagram which shows a transition from overlayers with square symmetry (and possible square dislocation patterns) to hexagonal symmetry. The phase diagram includes the experimentally observed clock-rotated phase. Local density approximation calculations suggested by the model show that a clean Ni(100) surface reconstructs from a bulk-terminated to a clock-rotated structure at biaxial compressive strains above $2.5\%$.

• Received 5 November 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.126101