STUDY OF THE MAGNETIC-PROPERTIES OF IRON(III) MOLYBDATE, BY SUSCEPTIBILITY, MOSSBAUER, AND NEUTRON-DIFFRACTION TECHNIQUES

A powder neutron diffraction study and subsequent line-profile analysis of the magnetic structure of iron(III) molybdate at 2 K indicate that it is a four-sublattice antiferromagnet containing four crystallographically distinct iron atoms in the space group P21/a (b unique). The results also confirm that the 2 K nuclear structure is the same as that found at room temperature by single-crystal X-ray diffraction. An analysis of covalency in iron(III) molybdate reveals a higher degree of covalency (Aσ2 + 2π2 + A82 = 9.15%) in the iron(III) to oxygen bonds than is found in the structurally related iron(III) sulfate (6.1%). A Mössbauer-effect study indicates that, at 11.8 K and above, iron(III) molybdate is paramagnetic with parameters typical of an octahedral high-spin iron(III) compound. Between 11.72 and 11.59 K both ordered and paramagnetic phases coexist. Below 11.72 K the Mössbauer spectrum shows the presence of spontaneous magnetic ordering with four inequivalent magnetic hyperfine fields. On three of the iron sites, the hyperfine fields are rather similar while the fourth site exhibits a significantly greater hyperfine field. The differences between various combinations of the four hyperfine fields show maxima at about 10 K. At 1.14 K the field values are 540, 529, 519, and 509 kOe. The isomer shifts for each site are similar at ca. 0.52 mm/s, and the quadrupole shifts are small at ca. ±0.01 mm/s. Magnetic susceptibility studies confirm that the material is paramagnetic above ca. 20 K, with an effective magnetic moment of 5.92 μB and a Curie-Weiss temperature of -55.6 K. The magnetic susceptibility shows a peak of ca. 10 K, the magnitude of which increases with decreasing applied field between 45.81 and 9.95 kG. The magnetization data also indicate spontaneous ferrimagnetic ordering for iron(III) molybdate below ca. 14 K with a maximum in the spontaneous moment between 9 and 10 K. This behavior confirms that, below ca. 20 K, iron(III) molybdate is a weak L-type ferrimagnet. At temperatures below 6 K, the magnetic susceptibility increases at high applied fields, indicating the presence of a spin-flop transition with a critical field of ca. 20 kG at 4.2 K. © 1982 American Chemical Society