Aluminous hematite: A mössbauer study

Aluminous hematites prepared in three different ways have been examined at 300K and 4.2K using the Mössbauer 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.