57Fe

57Fe is a stable isotope of iron with mass number 57. It accounts for about 2.1% of natural iron. The nucleus has a ground-state spin of 1/2 and a first excited state at 14.4 keV with spin 3/2. The 14.4 keV transition between these states is central to Mössbauer spectroscopy and enables extremely precise nuclear resonance measurements in solids.

In Mössbauer spectroscopy, a source containing 57Co decays to 57Fe in an excited state and emits a 14.4 keV gamma ray that can be resonantly absorbed by 57Fe nuclei in a sample. Because the gamma emission and absorption can occur without recoil in a solid, very sharp resonances are observed. The technique is sensitive to hyperfine interactions, yielding isomer shifts (information on electron density and oxidation state), quadrupole splitting (site symmetry and electric field gradient), and magnetic hyperfine splitting (internal or applied magnetic fields).

57Fe Mössbauer spectroscopy is widely used to study iron-containing materials across disciplines. Applications include characterization of iron oxides and silicates in geology, analysis of steel and other alloys in materials science and metallurgy, catalysis research, and investigation of iron-containing biological systems such as ferritin and heme proteins. Natural abundance may suffice for many samples, but researchers often use enriched material or optimized detection to enhance signal. The technique provides local structural and electronic information about iron sites, contributing to phase identification, oxidation-state determinations, and assessments of magnetic ordering.

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