Structure-property relationships in the R-type hexaferrites: Cation distributions and magnetic susceptibilities of MX2Fe4O11

The strong, and heretofore unexplained, dependence of the magnetic properties of the R-type hexaferrites MX₂Fe₄O₁₁ on heat treatment has been investigated by means of⁵⁷Fe and¹¹⁹Sn Mössbauer spectroscopy at 297 K.⁵⁷Fe Mössbauer spectra of samples of BaTi₂Fe₄O₁₁, BaSn₂Fe₄O₁₁, and SrSn₂Fe₄O₁₁, annealed at several temperatures consist of three partially resolved quadrupole doublet patterns. The pattern with the very large quadrupole splitting, i.e. 1.3 mm s^–1 E _Q 1.7 mm s^–1, is readily assigned to the trigonal bipyramidal site, and exhibits no dependence of its parameters, including the relative intensity, on heat treatment. The remaining two patterns are assigned to Fe³⁺ ions on octahedral sites, and exhibit relative intensities that are strongly dependent on heat treatment. The¹¹⁹Sn spectrum of BaSn₂Fe₄O₁₁ is a broad, apparently single line that, with the exception of verifying the presence of tin as Sn⁴⁺, provides little insight into the details of the structure. The distributions of the cations over all the lattice sites have been deduced from the Mössbauer spectra and compared with the magnetic susceptibility data. It is concluded that the cation distributions of the Sn-containing phase are not strongly dependent on heat treatment, and the magnetic properties exhibit a similar insensitivity to heat treatment. The Ti analogs exhibit a strong dependence of cation distribution on heat treatment, which is reflected in widely varying magnetic ordering temperatures, a variation that is confirmed by the incipient magnetic hyperfine splitting despite the strong magnetic frustration in the R-type hexaferrites.