Iron-57 and Antimony-121 Mössbauer-Effect Study of Several Spinel Antimonates

The iron - 57 and antimony - 121 Mössbauer - effect spectra of six iron - containing antimonate spinels, of general formula MII 12FeII 3SbV 3O24, where MII is divalent zinc, magnesium, cobalt, and nickel, and the spinel Ni10Fe6Sb2O24 have been measured and the 4.2 K iron - 57 results used to determine the cation distribution between the tetrahedral A and the octahedral B sites. If MII is diamagnetic zinc(II), all the iron(III) is located on the octahedral B site and the ordering temperature is ca. 1.35 K. In the orthorhombic spinel Mg12Fe3Sb3O24, two - thirds of the iron is on the tetrahedral A site and the ordering temperature is ca. 5 K. When MII is nickel(II) and either cobalt(II) or magnesium(II), there is a distinct preference for the nickel(II) to occupy the octahedral B site and, in so doing, to displace some additional iron(III) from these sites onto the tetrahedral A sites. The presence of magnetic cobalt(II) and nickel(II) cations increases the ordering temperature to ca. 200 K in these compounds. In each case the room - temperature spectra show lines corresponding to both tetrahedral and octahedral iron(III) quadrupole - split doublets with typical hyperfine parameters. Below the ordering temperature, each compound shows at least two iron - 57 Mössbauer - effect magnetic sextets with hyperfine fields at 4.2 K of ca. 450 kOe for the tetrahedral site and ca. 530 kOe for the octahedral site. In several instances, a small distribution in the hyperfine field is observed and attributed to the distribution of the cations on the near - neighbor sites. In each compound a few area percent of a paramagnetic impurity is observed in the magnetic spectra, and the nature of this impurity is discussed. The room - temperature antimony - 121 Mössbauer spectra of these materials show an isomer shift typical of antimony(V) and no hyperfine field at the antimony nucleus. Similar results are obtained for Zn12Fe3Sb3O24 and Mg12Fe3Sb3O24 at 4.2 K. However, the compounds containing magnetic MII dications show supertransferred hyperfine fields at the antimony, the magnitudes of which may be related to the nature and distribution of the dications