Mossbauer-Bauer-effect and x-ray-absorption spectral study of sonochemically prepared amorphous iron

The Mossbauer spectra of amorphous iron, prepared by using sonochemical methods, exhibit a broad magnetic hyperfine sextet at both 78 and 295 K. The spectra do not change with time if the amorphous iron is not exposed to oxygen or moisture. An analysis of the spectra with the method of Lines and Eibschutz yields average magnetic hyperfine fields of 29.1 and 25.9 T at 78 and 295 K, respectively. The corresponding moments of 1.9 mu(B), and 1.7 mu(B), agree well with values obtained from earlier magnetization studies and, further, provide strong experimental support for earlier calculations of the magnetic moments in amorphous iron. The observed average isomer shifts of 0.27 and 0.14 mm/s obtained at 78 and 295 K, respectively, correspond to a decrease in the s-electron density at the iron-57 nucleus as compared to that of alpha-iron, a decrease which is consistent with the decreased coordination number of amorphous iron. The similarity of the 295 K iron K-edge x-ray-absorption spectrum of amorphous iron and alpha-iron, up to 7130 eV, indicates that the d-electron density of states just above the Fermi level is similar in both forms of iron. The absence of structural details above 7130 eV in the spectrum of amorphous iron indicates, in agreement with multiple-scattering calculations, that long-range order does not extend beyond the third shell of neighbors in amorphous iron. Greatly reduced extended x-ray-absorption-fine-structure scattering is observed at the iron K edge of amorphous iron as compared to alpha-iron. An analysis of the weak observed scattering reveals both a decrease in the average coordination number from 14 in alpha-iron to 10 in amorphous iron, and an asymmetric radial distribution function of the iron neighbors in the first shell. This asymmetric distribution yields for amorphous iron a minimum iron-iron distance of 2.40 Angstrom and an average iron-iron distance of 2.92 Angstrom. [S0163-1829(98)08817-1]