  The heat of formation of Fe-Cr alloys undergoes an anomalous change of sign
at small Cr concentrations. This observation raises the question whether there
are intermetallic phases present in this composition range. Here we report the
discovery of several long-range ordered structures that represent ground state
phases at zero Kelvin. In particular we have identified a structure at 3.7% Cr
with an embedding energy which is 49 meV/Cr atom below the solid solution. This
implies there is an effective long-range attractive interaction between Cr
atoms. We propose that the structures found in this study complete the low
temperature-low Cr region of the phase diagram.
