PHASE DIAGRAM OF THE PLATINIUM-VANADIUM SYSTEM AS DEDUCED FROM DIFFUSE SCATTERING OF NEUTRONS. R. Caudron^1,2, D. Le Bolloc'h^1,2, A. Finel¹, ¹: ONERA, Laboratoire de Physique des Solides(OM) 29, avenue de la Division Leclerc, BP 72, 92322 Chatillon, FRANCE. ²: Laboratoire Leon Brillouin , CEA-CNRS CEN-Saclay, 91191 Gif sur Yvette FRANCE

The order-disorder behavior of alloys, and particularly their phase diagrams, is governed by the interaction energies between atoms, which can be extracted from diffuse scattering data in the disordered state. In situ elastic diffuse scattering of neutrons was performed on the Pt_(1-c)V_c system at two concentrations c = 1/4 and 1/8. For this system, the distribution of the diffuse intensity in the reciprocal space is very sensitive to the concentration and the temperature. Through inverse Monte-Carlo simulations, sets of effective pair interactions (EPI) were deduced within the Ising Model for the two different concentrations. Surprisingly, despite quite different SRO patterns, the EPI are almost concentration and temperature independent. Hence, the two very different diffuse intensity maps, at c = 1/4 and 1/8 are well reproduced with a unique set of interactions. That set allowed us to describe also properties of the ordered states: in particular, the degeneracy effects between long periods at c = 1/4 and the transition temperatures T_c are well reproduced. Such sa situation, where the interactions are independent of the concetrtation and the temperatire, The predicted phase diagram is discussed. It is in a good agreement with the known transition temperature for the ordered states at c = 1/4 and 1/8. Furthermore, we predict the existence, at very low temperature, of a new phase of composition c = 1/6 which has never been observed until now.