Identification of an incommensurate FeAl2 over-layer on FeAl(110) using x-ray diffraction and reflectivity

FeAl, like NiAl, crystallizes in the CsCl structure. Consequently, the bulk (110) planes contain equal amounts of Fe and Al distributed as interlocking rectangles. Unlike the NiAl(110) surface, which retains the (1 x 1) in-plane symmetry of the bulk, FeAl(110) reconstructs to form an ordered, incommensurate overlayer. The reconstructed layer is discerned from x-ray diffraction rods at half-order positions along the [1(1) over bar0$] direction, and displaced +/-0.2905 from integer positions along the [001] direction. Peak widths reveal excellent long-range order. Specular reflectivity measurements above and below the Fe K alpha edge can be reproduced using a model containing a single reconstructed overlayer with an Fe:Al ratio of 1:2, consistent with FeAl2.