Local atomic arrangements in Fe63.2Ni36.8 invar from diffuse X-ray scattering measurements

X-ray measurements show that Fe63.2Ni36.8 has a slight tendency toward L1(0 ) and/or L1(2) ordering with a preference for weakly ordered {100} platelet s. This bias for Fe-Ni first neighbor pairs decreases the number of Fe atom s with eight or more Fe nearest neighbors. Such Fe rich regions are strongl y correlated to the alloy's Invar properties. The mean separation of Fe-Fe first neighbor pairs is expanded relative to the average lattice, and the a mount of expansion increases upon cooling from 293 to 60 K. This expansion supports the large moment-large volume explanation for the Invar effect. [S 0031-9007(99)08748-7].