CONCENTRATION VARIATION OF THE EFFECTIVE PAIR INTERACTIONS MEASURED ON THE PT-V SYSTEM - EVALUATION OF THE GAMMA-EXPANSION METHOD

We present a detailed study of interactions in two alloys, Pt3V and Pt 8V, and preliminary results on Pt4V. In situ neutron scattering experi ments were performed in order to measure the short range order paramet ers in the disordered phase for the two concentrations. We will show t he drastic effect of the concentration on the short range order (SRO), particularly in Pt8V, where the diffuse intensity maxima do not peak at special points of the fee lattice. Using inverse Monte Carlo (IMC) simulations and the gamma expansion method (GEM), two sets of effectiv e pair interactions (EPI) were deduced from the experimental SRO param eters within the Ising model. Surprisingly, despite quite different SR O patterns, the EPI seem nearly concentration independent. We will sho w, using MC simulation, that the two very different diffuse intensity maps are well reproduced with the same interaction set, The concentrat ion independence of the EPI allowed us to describe also properties of the ordered states and more generally the whole phase diagram. In part icular, wt predict the existence, at very low temperature, of a new ph ase of stoichiometry A(5)B which has never been observed until now.