Mössbauer Study of Hyperfine Fields and Isomer Shifts in Fe 4 N and ( F e , N i ) 4 N

Mössbauer measurements of ${\mathrm{Fe}}^{57}$ were made on ferromagnetic ${\mathrm{Fe}}_4$N, which has a face-centered cubic arrangement of iron atoms with nitrogen at the body-center position. The hyperfine fields are 345 koe for the corner Fe and 215 koe for the three face-center Fe, approximately proportional to their magnetic moments, $3{\mu }_B$ and $2{\mu }_B$. The isomer shifts, measured against a stainless steel source, are 0.30 mm/sec for the corner Fe and 0.45 mm/sec for the face-center Fe. These values are in line with their proposed electronic configurations of $3d^{7}4s$ and $3d^{8}4s$, which are derived on the assumption that the nitrogen at the body-center position acts as an electron "donor" to the face-center Fe. The Mössbauer spectra of (${\mathrm{Fe}}_{3.6}$,${\mathrm{Ni}}_{0.4}$)N and (${\mathrm{Fe}}_3$Ni)N are consistent with their ordered structures in which Ni replaces the corner Fe preferentially.