Structure of the clean NiAl(110) surface and the Al2O3/NiAl(110) interfaceby 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 R-Ni/Al = 0.16 +/- 0.01 Angstro m for the clean surface and R-Ni/Al = 0.18 +/- 0.02 Angstrom for the oxide covered surface. On the clean surface the Al sites are expanded by +3.8% (o utwards) 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 p ositions has been found, while the Ni atoms remain (-0.9%) at the bulk posi tion. On both cases, an ideal surface stoichiometry (1:1) was obtained. How ever, 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. (C) 2001 Elsevi er Science B.V. All rights reserved.