STUDY OF ANHYDROUS IRON(III) SULFATE BY MAGNETIC-SUSCEPTIBILITY, MOSSBAUER, AND NEUTRON-DIFFRACTION TECHNIQUES

A powder neutron-diffraction study and subsequent line-profile analysis of the nuclear and magnetic structure at 4.2 K of monoclinic anhydrous iron(III) sulfate indicate that it is a two-sublattice antiferromagnet. Each sublattice is made up of crystallographically distinct iron atoms in the space group P21/n (b-unique). The nuclear structure at 4.2 K is essentially the same as that found at room temperature by single-crystal X-ray analysis, An analysis of covalency in the material indicates that the degree of covalency is consistent with that found for other iron(III) ions octahedrally coordinated by oxygen. A Mossbauer effect study indicates that above 28.8 K this compound is paramagnetic with parameters typical of an octahedral high-spin iron(III) compound. Below 28.7 K the Mossbauer spectrum reveals the presence of spontaneous magnetic ordering with nonequivalent internal hyperfine fields on the two sublattices. The difference between these fields reaches a maximum at 23 K and decreases until, at 4.2 K, the field on each sublattice is essentially the same at ca. 550 kOe. The two sublattices have essentially the same isomer shifts while the quadrupole shifts are similar in magnitude but opposite in sign. Applied-field Mossbauer effect studies further support an antiferromagnetic coupling model. Magnetic susceptibility studies confirm that the material is paramagnetic with an effective magnetic moment of ca. 5.92 between 298 and 40 K. Below this temperature the magnetic susceptibility increases sharply to a maximum at 23 K and then decreases rapidly. This behavior results because of the nonequivalent magnitude of the spontaneous magnetization on each sublattice between 28 and 4.2 K. A magnetic superexchange coupling model is proposed to explain the magnetic nonequivalence of the two sublattices over this temperature range. © 1979, American Chemical Society. All rights reserved.