Critical behavior of the order-disorder phase transition of the (2 x 2)-2H structure on Ni(111) eras investigated for the pure system and in the presence of 0.3% -3% of a monolayer of preadsorbed atomic oxygen. Depending on oxygen concentration, the values of the critical exponents p, 7, and v change from those of the four-state Potts universality class to Ising-like The role of local defects is addressed in our study of the critical properties of the order-disorder phase transition of the (2 x 2) structure of atomic H on the Ni(ill)

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Critical behavior of the two-dimensional order-disorder phase transition of the (2 x 2)-2H structure on Ni(ill) was studied by high-resolution low-energy electron-diffraction experiments, both for the pure system and with atomic oxygen, at concentrations between 0.3% and O'Fo of a monolayer added as an impurity. By quantitative profile analysis of several superstructure spots as a function of temperature the critical exponents P, p, and v were determined over a range in the reduced temperature, t = (T-T,)/T"between 0.001 and 0.1. At smaller values of~t~the system is finitesize limited. Crossover between four-state Potts behavior further away from T, and exponents close to those of the two-dimensional Ising class were found. The crossover temperature depends systematically on the concentration of oxygen. We conclude that oxygen atoms seem to act like quenched impurities on this surface, similar to local defects already present at concentrations of (10 of a monolayer on the clean surface.

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