Studies of the crystal structure and crystal chemistry of titanomaghemite

A natural single crystal of titanomaghemite from Pretoria, South Africa, containing 8.62 wt% Ti and 54.36 wt% Fe as determined by electron-microprobe analysis, has been studied by X-ray diffraction and Mössbauer spectroscopy. A crystal-structure parameter refinement of this titanomaghemite was undertaken, and values for the atom positions, anisotropic thermal parameters, and bond lengths are reported (R = 0.0235). The lattice is primitive cubic (a = 8.341(1) Å) with space group P4₃32 (or the enantiomorphous P4₁32). Studies of this sample, and of two powder samples of synthetic end-member maghemite, using ⁵⁷Fe Mössbauer spectroscopy at 300 K and 4.2 K and in a longitudinal applied magnetic field of 4.2 T, indicate that for the end-member γ-Fe₂O₃ samples, cation vacancies are restricted to octahedral sites. Combined single-crystal X-ray studies and Mössbauer spectroscopy of the titanomaghemite have given the following structural formula:
Fe3+0.96□0.04[Fe2+0.23Fe3+0.99Ti4+0.42□0.37]O2−4,
where □ represents a cation vacancy. As the above formula shows, there are cation vacancies on both tetrahedral and octahedral sites in this titanomaghemite, the majority being on the octahedral sites. We believe the evidence of the Mössbauer spectra points to Fe²⁺ on the octahedral site. Mössbauer parameters are reported for the end-member γ-Fe₂O₃ samples and the titanomaghemite, and the cation and vacancy distributions on the sites in the latter are discussed in the light of previously published models.