Stability of ultrathin alumina layers on NiAl(110)

By observing with low-energy electron microscopy whether individual alumina islands grow or shrink for different substrate temperatures and O(2) pressures, we determine the stability of thin oxide layers on the NiAl(110) surface. At each temperature, a well-defined O(2) pressure exists where islands do not change in size. Yet we conclude that the oxide cannot be in thermodynamic equilibrium With O(2) gas and NiAl bulk, because the O(2) pressures needed to attain this state are 20 orders of magnitude higher than expected. We discuss what kinetic processes can lead to the observed steady state, where the O(2) pressure needed for stability differs greatly from thermodynamic predictions.