Chemical order of manganese-antimony liquid alloys obtained experimentally by the neutron "null matrix" method

The Bhatia-Thornton concentration-concentration partial structure factor S-CC(q) is a strong indicator of the order in liquid alloys. Manganese is one of the four metals, which has a negative neutronic scattering length, while the antimony one is positive. Thus for a defined composition (Mn60Sb40) the linear combination of the two neutronic scattering lengths weighted by the atomic compositions is zero (zero alloy). We present here the results of neutron diffraction on the Mn60Sb40 "null matrix" alloy at 950degreesC, which is proportional to the Bhatia-Thornton S-CC(q) ("null matrix" method). The total structure factor of Mn40Sb60 at 800degreesC has also been measured. The main peak of the experimental S-CC(q) is a proof of a strong chemical order in this alloy. This order is confirmed in the real space by the Fourier transform of the structure factor. To interpret our experimental results, one generally uses effective potentials determined with the pseudopotential formalism. But transition metal pseudopotentials are not easy to handle especially in alloys. We used different simple effective potential models: hard spheres with constant diameters, hard spheres with composition dependent diameters, shouldered hard spheres models. We first show that the hard sphere model cannot reproduce the experimental results if we postulate that the hard sphere diameters do not change on alloying. Then we fit the hard sphere diameters on the experimental structure factor of the alloy. This schema can correspond physically to a charge transfer between the two components. We show that it can no more explain the experiment. The addition of attractive and (or) repulsive contributions to the different interatomic potentials give satisfactory results. The best results are obtained with attractive contributions between unlike atoms and repulsive ones between identical atoms. We have also shown that the parameters obtained for Mn60Sb40 give also good results for the structure factor at a second composition: Mn40Sb60.