Abstract: | The magnetic interaction in the structural units Fe2O7]8?, built of two corner-sharing FeO4 tetrahedra, in Na8Fe2O7 () has been studied by magnetic susceptibility measurements (4.2–500 K). An exchange integral of ?37 K is obtained by comparison of the experimental values and the calculated ones using a Heisenberg-Dirac-Van Vleck-type Hamiltonian ? = . The hypothesis of magnetically isolated Fe2O7]8? groups is corroborated by Mössbauer spectroscopy between 1.5 and 77 K. The susceptibility measurements of the solid solutions Na8Fe2?xMxO7 (M = Al, Ga; 0 ≤ x ≤ 0.2 for Al; 0 ≤ x ≤ 2 for Ga) leads to the same conclusion of the existence of isolated Fe3+Fe3+ pairs in Na8Fe2O7. The type of substitution of Fe by Al or Ga is determined; homonuclear Fe3+Fe3+ and M3+M3+ pairs and heteronuclear Fe3+M3+ pairs are formed. |