  The energy of segregation of Cr from the bulk to the topmost surface layer in
dilute Fe-Cr alloys is endothermic but small. On the other hand, the
segregation energy from the bulk to the subsurface layer is not only
endothermic but relatively large. Thus, the subsurface layer presents a huge
barrier, not only for the segregation of Cr from the bulk to the surface, but
also for its diffusion from the surface into the bulk. This means that the
topmost layer alone cannot be used to determine the barrier for Cr segregation
(the segregation energy), which is determined in these alloys by the subsurface
layer. We present the results of our ab initio electronic structure
calculations on the segregation of Cr as an impurity in Fe, which shed some
light on the origin of this anomalous behavior. We find that the interaction of
a Cr atom situated in the subsurface layer, in contrast to those in the other
layers, is quite complex with its nearest neighbor Fe atoms due to the
magnetism of both Fe and Cr, leading to a highly unfavorable electronic
structure. This in turn leads to a large endothermic energy of solution of Cr
in the subsurface layer. These results are important for a proper understanding
of the properties of the technologically important class of Fe-Cr steels with
low concentrations of chromium.
