Epitaxial alloyed films out of the bulk stability domain: Surface structure and composition of Ni3Al and NiAl films on a stepped Ni(1 1 1) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Auger Electron Spectroscopy (AES) Ni–Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(1 1 1) surface. The surface structure and composition of the thin epitaxial Ni3Al and NiAl films, obtained respectively below and above a critical Al initial coverage θc, differ markedly from those of corresponding bulk alloys. 3Al ordered films form in a concentration range larger than the stability domain of the L12 Ni3Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases. o appears that the value of θc depends on the morphology of the Ni(1 1 1) substrate, increasing from θc = 4.5 ML for a flat surface to θc = 10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni–Al interdiffusion.