Structure of the clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface by measurements of crystal truncation rods

The clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface have been investigated by synchrotron X-ray diffraction experiments. In the case of the oxide surface the analysis of the NiAl(1 1 0) crystal truncation rods (CTR) provide information about the interface between the Al2O3 film and the NiAl substrate. The analysis of the CTR-data shows clearly a rippled Ni–Al topmost surface with an amplitude value of RNi/Al=0.16±0.01 Å for the clean surface and RNi/Al=0.18±0.02 Å for the oxide covered surface. On the clean surface the Al sites are expanded by +3.8% (outwards) and the Ni sites are contracted by −3.2% (inwards) respect to the unrelaxed interlayer separation. For the oxide covered surface an increase of the expansion of the outermost Al atoms (+7.3%) relative to their bulk positions has been found, while the Ni atoms remain (−0.9%) at the bulk position. On both cases, an ideal surface stoichiometry (1:1) was obtained. However, some intermixing (chemical disorder) of one specimen in the sites of the other and vice versa was present (less than 4%). This chemical disorder was not enhanced by the presence of the Al2O3 overlayer. Neither rippling nor oscillatory relaxation in deeper layers was detectable.