Effect of oxygen on the stability of 1-D Lps in Cu3Pd

Abstract

The variety of ordered phases exhibited by Cu-Pd alloys has led many investigators to study it both experimentally and theoretically. Recently, we have determined the Cu-rich portion of the Cu-Pd phase diagram (Fig. 1) and indicated that the region denoted as 1-d LPS actually represents a large number of single phase fields of long period superstructures (LPS) separated by narrow two phase regions in agreement with theoretical calculations. Each single phase field would correspond to an LPS with a characteristic modulation period, which can be measured from the splitting of the 001 reflection in diffraction patterns. High resolution electron microscopy (HREM) studies on Cu-Pd alloys have revealed that the antiphase boundaries (APB) associated with the LPS formed at low temperatures exhibit a sharp character, while those in LPS equilibrated at high temperatures are diffuse. In a recent theoretical study on LPS, Ceder et al. pointed out that the pair interaction potential in reciprocal space, V(k), associated with Cu-Pd alloys shows a rather deep minimum which arises as a result of flat sections on the Fermi surface. Ordering concentration waves located away from the minimum of V(k) would quickly disappear at higher temperatures because of their higher contribution to the configuration energy, thus leading to the formation of a smooth modulation profile. Consequently, in selected area diffraction patterns (SAD), only first order satellites are expected. On the other hand, at lower temperatures, where the contribution from the ordering concentration waves away from the minimum in V(k) is also significant, higher order satellites should be observable in SAD patterns.