Internal Fields at the Two Sites of the Spinel System Ni1+xGexFe2−2xO4 by Mössbauer Effect

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Abstract
In the study of ferrites, some authors have found two Mössbauer resonances of Fe3+ in tetrahedral (A) and octahedral (B) sites. The lower value of the internal field at the nucleus of Fe3+ in A sites has been attributed to the dipolar field, resulting from deviations from cubic symmetry, and to the more covalent nature of iron‐oxygen bonds. When the A site behaves as a perfect tetrahedron, the quadrupole interaction being absent, the dipolar field is excluded and the decrease in the field HA can be attributed to the effect of covalence alone. Those requirements are fulfilled in the spinel system Ni1+xGexFe2−2xO4. No quadrupole interaction is evident in any of these spectra. The lower value of HA in all cases is attributed to the covalent effects in Fe3+–O2− bonds in A sites. The variation of HA and HB with x at 300°K is shown to be the effect of Curie temperature. At 80°K HB is independent of x, whereas HA decreases with an increase in x. This diminution in HA is attributed to the modification brought about by the presence of Ge4+ in A sites making the Fe3+–O2− bonds more covalent. This hypothesis is supported by the lower isomer shift of A sites.