EXPERIMENTAL AND THEORETICAL-STUDY OF THE TEMPERATURE AND CONCENTRATION-DEPENDENCE OF THE SHORT-RANGE ORDER IN PT-V ALLOYS

We present a detailed theoretical and experimental study of the short- range order in the Pt1-cVc system at two concentrations (c = 1/4 and c = 1/9). In situ neutron-scattering experiments were performed in orde r to measure the short-range-order parameters in the disordered phase. We found a drastic effect of the concentration on the short-range ord er: in Pt3V, the diffuse intensity is spread along the (1k0) direction s with maxima at the (100) positions, despite the stability at low tem perature of a DO22 phase. In contrast, the diffuse intensity in Pt8V d isplays a splitting around the (100) positions with incommensurate max ima. Through inverse Monte Carlo simulations the two experiments yield , within the Ising model, two sets of effective-pair interactions. Des pite quite different short-range-order patterns, the interactions seem nearly concentration independent with a dominant first-neighbors inte raction V-1. This concentration independence allows us to predict the ordered states and the ordering temperatures. In particular, at low te mperatures, these interactions stabilize a new phase of composition AS B, which to our knowledge has not been observed until now. Finally, we analyze the origin and behavior of the incommensurate split peaks in Pt8V within a high-temperature expansion and show analytically that th e splitting is due to a large decrease of the influence of V-1 on the short-range order as the concentration and/or the temperature decrease s. This analysis shows also that the splitting distance should decreas e with increasing temperature, in agreement with our Monte Carlo simul ations, and in contrast with all the other alloys which have already b een investigated, either experimentally or theoretically. More general ly, we discuss the origin of the temperature behavior of a splitting d istance in relation with the location in q space of the incommensurate maxima. Using very simple arguments, we show, provided the restrictio n that the first-neighbor interactions are dominant, that the splittin g distance increases or decreases with increasing temperature dependin g on whether these maxima lie along the (1k0) axis or not.