Variable temperature neutron diffraction was carried out on mesoporous α-Fe2O3 (hematite) with a mean pore diameter of 38.5 A. Data were directly compared to measurements carried out on bulk hematite. Unlike bulk hematite, the mesoporous material does not undergo a spin−flop transition from a weak ferromagnet to a pure antiferromagnet (Morin transition, TM = 259.1(2)). Instead, the material remains a weak ferromagnet down to 2 K with the magnetic moments staying perpendicular to the R3c [111] ([003] in the hexagonal cell) direction rather than realigning (to) almost parallel to this direction. The angle of the magnetic Fe3+ moments to the [111] direction in the antiferromagnetic state also accurately was obtained for bulk hematite. Using magnetic hysteresis measurements, the canting angle responsible for weak ferromagnetism within the ab planes (hexagonal setting) was deduced for the mesoporous material at 12 K and compared to the angle made in bulk hematite above TM.

Neutron Diffraction Study of Mesoporous and Bulk Hematite, α-Fe2O3

Mössbauer study of aluminum-substituted hematites

Aluminous hematites prepared in three different ways have been examined at 300K and 4.2K using the Mossbauer technique. The results indicate significant differences between the behaviour of aluminous hematites that have been subjected to high temperatures (>600° C) and those which have not. The magnitude of the room temperature quadrupole splitting of the former increases with aluminium content, approaching at ∼16 mole percent substitution the value (−0.22 mm/s) exhibited by all of the low temperature specimens. This variation may be explained qualitatively in terms of a preferential c-axis contraction of the lattice upon incorporation of aluminium, which does not occur unless a c-axis defect structure is removed by subjection of the hematite to high temperatures. The solid solubility limits of high and low temperature hematites (∼15 mol % and ≥19 mol % respectively) also differ, as do the room temperature decreases in hyperfine splitting (−0.82 kOe/mol % Al and −0.86 kOe/mol % Al). At 4.2 K only low temperature hematite exhibits a decrease in hyperfine splitting with increasing Al content (−0.40 kOe/mol % Al). The absolute values of the recoil free fractions of hydrothermally prepared aluminous hematites have been determined at 4.2 K (0.70±0.02 — pure hematite, 0.82±0.02, 14±2 mol % Al substitution) and exhibit a similar increase with Al content to that previously observed for aluminous goethites. The room temperature recoil free fraction of pure hematite has been measured to be 0.64±0.02. The effects of particle size on both hyperfine splitting and recoil free fraction have been investigated.

Aluminous hematite: A mössbauer study

Mossbauer spectra of 15 hematites with Al substitutions between 0 and 10 mole % were taken at room temperature. X-ray powder diffraction indicated dimensions of these hematites in the c-direction to range upwards from 27 nm to crystals large enough to show no line broadening. The Mossbauer spectra showed that magnetic hyperfine fields decreased both with increasing Al-for-Fe substitution and with decreasing crystal size. These relationships indicate that hyperfine field variations cannot, as has been done in the past, be unequivocally related to Al substitution alone. Hyperfine field reductions were paralleled by Mossbauer line broadening due to hyperfine field distributions. Only the hematites heated to 1000°C showed a significant variation of quadrupole splittings with Al substitution. No dependence of quadrupole splitting on crystal size was observed, indicating no detectable distortion of coordination polyhedra in the particle size range studied.

B. Molnàr

Papers

Neutron diffraction study of the (1-x)αFe2O3xAl2O3 system