Crystallochemical characterization of magnetic spinels prepared from aqueous solution

Abstract

The crystallochemical characterization of magnetic spinels prepared from aqueous solution has been studied primarily by electron microscopy and ⁵⁷Fe Mössbauer spectroscopy. Two synthetic routes have been investigated: method 1; partial oxidation of Fe^II solutions in the presence of NO^–₃ at 100 °C, and method 2; reaction of hydrated Fe^III oxide (ferrihydrite) with Fe^II ions at room temperature, pH = 7, 8 and 9. Both routes gave small (10–60 nm) irregular magnetite (Fe₃O₄) particles that were readily oxidised to maghemite (γ-Fe₂O₃). The reaction proceeded via a green-rust intermediate and anions such as inorganic phosphate (P_i) and SO^2–₄ reduced the rate of transformation. Spinel formation at room temperature was severely inhibited by P_i levels of 1 mol %, whereas 20–30 mol % P_i was required for retardation of crystallization at 100 °C. Intermediate levels (5–10 mole %) resulted in morphological enhancement of the octahedral crystal habit. A similar effect was observed in the presence of SO^2–₄ and at neutral pH in method 2.