## 2 Experimental Techniques and Methodology  
Three techniques are now described that can be combined to study the phenomenon described in the present paper (for more details see [2]). First, the study of the isothermal sections of phase diagrams is essential in order to assess the shape of the homogeneity domain. They can be collected from the literature or, when needed, one's own determination can be made. Alloys can be synthesized by arc-melting of the pure elements followed by a convenient annealing at the required temperature and quenching. The characterization of the nature of the present phases is done by X-ray diffraction, and the composition of each phase is determined by electron probe micro-analysis (EPMA). In the present work, Re-Cr-Co alloys were obtained by arc melting of the pure elements (pellet obtained from the powders of Re and Cr, Alfa Aesar, 99.99%, and Co, Aldrich, 99.99%) followed by an annealing at 1150 \({}^{\circ}\)C in evacuated sealed silica tubes for six weeks. Due to difficulties in reducing the inhomogeneities related to the solidification in the Re-richer sample at this temperature, this sample was submitted to a first annealing treatment at 1500 \({}^{\circ}\)C under argon for 8 h in an induction furnace before the annealing at 1150 \({}^{\circ}\)C. The EPMA measurement was carried with a Cameca SX 100 apparatus using K\(\alpha\) lines for Cr and Co and L\(\alpha\) line for Re. In the present study, devoted to site reversal, it is obviously of primary importance to be able to experimentally obtain the site occupancies of the three atoms on the different sites of the crystal structure as a function of composition. X-ray or neutron diffraction alone cannot be used to solve this type of problem since, from one diffraction dataset, one can obtain only one occupancy parameter per site to match the average scattering density. If one has to obtain, on each site, the two site occupancies necessary to define the structure when three elements are present together at the same position, one has to use two  
Figure 2: Schematized isothermal section of a system _A_–_B_–_C_ in which the compound _ACx_ has a tendency to be substituted on both sites by the \(B\) atom, forming stable or metastable _BCx_ and _ABx_ compounds. diffraction datasets in which the diffraction contrast is different. Ternary site occupancies can therefore be obtained in many cases by combined Rietveld refinement of neutron (ND) and X-ray (XRD) diffraction data. This is illustrated in Figure 3 in the case of the Mo-Re-Ni system. The red lines show the locus of compositions giving the same neutron scattering density at a given site. All the site compositions on the line would give the same agreement with the diffraction data in a structure refinement if the neutron diffraction only was used. The same lines can be drawn for X-ray and it is evident that only one composition can result in a good refinement of both XRD and ND data. One may appraise how accurately the exact site composition can be determined in this particular system by the fact that the lines cross orthogonally. Note that this is not always the case and the accuracy depends on the relative values of the Fermi lengths and atomic numbers of the three present elements. In Figure 3, the example of the Mo-Re-Cr system is also presented for which the lines are almost parallel and the determination of ternary site occupancies would be much less accurate. This technique was used in the case of Zr-W-Re [3], Mo-Re-Ni [4], and Re-Cr-Co systems (this work) in order to obtain the ternary site occupancies for the \(\chi\) and the \(\sigma\) phases. In each case the neutron diffraction measurements were performed at Laboratoire Leon Brillouin (common laboratory CEA-CNRS) in Saclay, France using the instrument 3T2. The powder sample (typically 6-8 g, \(<\)63 \(\mu\)m) was placed in a vanadium cylinder (diameter 6 mm) and the measurement was performed at room temperature with Debye-Scherrer geometry in the presence of five detectors.