Surface energy and the early stages of oxidation of NiAl(110)

We have studied the (110) surface of NiAl, an ordered alloy of B2 structure , using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces were investigated, with oxygen coverages up to 1.5 ML (1 ML = 1 O-atom per surface metal atom). In order to compare the energies of the oxidized structures, which comprise different numbers of metal and oxyg en atoms, one has to take account of the chemical potentials of the Ni, Al and O. To this end, for each system, we have applied simple analytic models to study their surface energy as a function of temperature, alloy stoichio metry (assumed to be near 50-50) and oxygen partial pressure. The calculati ons predict how, at oxygen pressures just above the threshold for decomposi ng NiAl, the clean surface should be coated by an alumina layer, with the c onsequent depletion of Ni near the surface. Varying stoichiometry has the r elatively minor effect of shifting the crossovers in oxygen pressure at whi ch different oxidized surfaces become stable. (C) 2001 Elsevier Science B.V . All rights reserved.