Study of the reduction and reoxidation of substoichiometric magnetite M. Gotic ´ * , G. Košc ˇec, S. Music ´ Division of Materials Chemistry, Ruder Boškovic ´ Institute, P.O. Box 180, HR-10002 Zagreb, Croatia article info Article history: Received 7 October 2008 Received in revised form 24 October 2008 Accepted 27 October 2008 Available online 6 November 2008 Keywords: Magnetite Maghemite Hematite Stoichiometry Mössbauer FT-IR abstract The commercial magnetite powder was characterised as substoichiometric magnetite (Fe 2.93 O 4 ) with a small hematite fraction (7 wt%). It consisted of micrometer-sized particles of regular (octahedral) and irregular morphologies. Reference sample was subjected to reduction by hydrogen gas and reoxidation at a temperature of up to 600 °C. The oxidation experiments were performed in an oxygen stream. Ref- erence sample oxidised to a maghemite–hematite mixture, whereas the fraction of each phase was very sensitive to the oxidation temperature. XRD and FT-IR spectroscopy indicated the superstructure charac- ter of obtained maghemite samples with the ordering of cation vacancies in the maghemite crystal lat- tice. Mössbauer spectroscopy was very sensitive to magnetite stoichiometry, however, it could not detect less than 10 wt% of hematite in the maghemite–hematite mixture. In reduction experiments, con- ditions for the reduction of reference sample to stoichiometric magnetite (Fe 3.00 O 4 ) were found. The reduction was performed by hydrogen gas under static conditions (375 °C, 150 min). Under this static condition the relatively high amount of water condensed inside the evacuated quartz tube. The formation of water is due to the chemisorption of hydrogen molecules on the iron oxide, which then dissociate from iron oxide generating an intermediate hydroxyl group according to the general equations: 2O 2 ðsÞþ H 2 ðgÞ! 2OH  ðsÞþ 2e  and2OH  ðsÞ! O 2 ðsÞþ à a þ H 2 OðgÞ; where (s) signifies a solid (hema- tite or substoichiometric magnetite), (g) a gas phase and à a an anionic vacancy formed in hematite or substoichiometric magnetite. Thus, in order to ensure a good reduction condition, water vapour was removed from the quartz tube and the tube was refilled with hydrogen gas every 30 min. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Synthetic iron oxides [1–5] (a group name for iron oxyhydrox- ides and oxides) are important materials with numerous applica- tions. Amongst them, magnetite (Fe 3 O 4 ) and maghemite (c- Fe 2 O 3 ) are ferrimagnetic at room temperature and they possess un- ique electric and magnetic properties. They are widely used as material in catalysis, magnetic separation, ceramics processing, en- ergy and magnetic data storage, and as protection against micro- wave radiation. Moreover, both magnetite and maghemite have been extensively investigated for application in biomedicine [6,7]. For example, they have been recognised as suitable magnetic materials for MR imaging, drug delivery, drug targeting, cell label- ling and magnetic separation, as well as a hyperthermia agent. Magnetite has better magnetic properties than maghemite, how- ever, magnetite tends to oxidise easily due to the presence of Fe 2+ , whereas maghemite is chemically and physically very stable. The oxidation of magnetite deteriorates its magnetic properties. The common process for the preparation of maghemite starts from acicular precursor crystals of either non-magnetic a-FeOOH (goe- thite) or non-magnetic c-FeOOH (lepidocrocite) [8]; these are dehydrated directly to a-Fe 2 O 3 (hematite) and further reduced to Fe 3 O 4 (magnetite). Magnetite is then carefully oxidised to yield fer- rimagnetic acicular c-Fe 2 O 3 (maghemite) particles. Schematically the process can be shown as follows: a  FeOOH ! dehydration 300  C a  Fe 2 O 3 ! reduction in H 2 350  C Fe 3 O 4 ! oxidation in air 300  C c  Fe 2 O 3 Generally, the maghemite particles thus obtained showed high saturation magnetization and high coercivity due to their shape anisotropy. However, each of the above steps is important, since the magnetic properties of the final product depend on particle size, morphology, crystallinity and present impurities. Hematite is the most frequent impurity accompanying maghemite. The pres- ence of hematite may significantly disturb the magnetic properties of maghemite. Magnetite (Fe 3 O 4 ) has an inverse spinel structure in which cat- ions occupy tetrahedral and octahedral sites in the face-centred cu- bic close-packed oxygen lattice [9,10]. Stoichiometric magnetite contains eight Fe 3+ at tetrahedral (8a) or A-sites and 16 cations (8Fe 3+ + 8Fe 2+ ) at octahedral (16d) or B-sites. Fe 2+ ions in stoichi- ometric magnetite easily oxidise to form the structure deficient 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.10.048 * Corresponding author. Tel.: +385 1 4561 123. E-mail address: gotic@irb.hr (M. Gotic ´). Journal of Molecular Structure 924–926 (2009) 347–354 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc