Local Atomic Arrangements in Fe{sub 63.2} Ni{sub 36.8} Invar from Diffuse X-Ray Scattering Measurements
X-ray measurements show that Fe{sub 63.2} Ni{sub 36.8} has a slight tendency toward L1{sub 0} and/or L1{sub 2} ordering with a preference for weakly ordered {l_brace}100{r_brace} platelets. This bias for Fe-Ni first neighbor pairs decreases the number of Fe atoms with eight or more Fe nearest neighbors. Such Fe rich regions are strongly correlated to the alloy{close_quote}s Invar properties. The mean separation of Fe-Fe first neighbor pairs is expanded relative to the average lattice, and the amount of expansion increases upon cooling from 293 to 60thinspthinspK. This expansion supports the large moment{endash}large volume explanation for the Invar effect. {copyright} {ital 1999} {ital The American Physical Society}