LOCAL AND NONLOCAL ISOMER-SHIFTS IN BCC FE-X ALLOYS (X=AL,SI,GA,GE)

We measured Mossbauer spectra from bcc alloys of compositions near Fe3Al, Fe3Si, and Fe3Ge, and reanalyzed older data from Fe3Ga. All four alloys developed some D0(3) chemical order, and their Mossbauer spectra were qualitatively similar. The local (first-nearest-neighbor) could be distinguished from the nonlocal (beyond the first-nearest-neighbor) contributions to the Fe-57 isomer shifts from the solute atoms owing to differences in their hyperfine magnetic fields. The nonlocal isomer shift contributions correlated best to the valence of the solute atom, and not to the atomic volume. The local isomer shift contributions were best attributed to a loss of Fe 3d electrons at the solute sites.