Structural properties of Mn-substituted hercynite

Spinels are compounds that have the general formula AB2O4 where A and B are metallic cations. The formal charges of A and B are strictly specifi ed and they are a criterion of the spinel type. There are three types of spinel compounds, the so-called 2-3, 4-2 and 6-1 spinels, where the numbers indicate the formal charges of the cations. The fi rst cation of each pair occupies a tetrahedral site (Td), while the second one is located in the octahedral position (Oh) [1]. Hercynite (FeAl2O4) is a 2-3-type spinel with a normal cation distribution, which means that all of the divalent ferrous ions occupy tetrahedral (Td) positions, whereas the Al3+ cations occupy octahedral (Oh) sites. Nevertheless, this order in the cation arrangement is impaired at temperatures above absolute zero causing cation exchange in the real spinel-type compounds between Td and Oh sites [1–3]. This disorder is described by the inversion parameter i, which stands for the fraction of trivalent ions in the Td interstices. The formula of such a mixed spinel can be expressed as (Fe1–xAlx)(FexAl2–x)O4. The inversion parameter ranges from 0 (normal spinel) to 1 (inverse spinel). Typical representatives of inverse spinels are magnetite (Fe3O4) [4] and jacobsite (MnFe2O4) [5]. As a typical normal spinel compound, FeAl2O4 exhibits a cubic structure Fd-3m (space group 227) Structural properties of Mn-substituted hercynite Ilona Jastrzębska, Wiktor Bodnar, Kerstin Witte, Eberhard Burkel, Paweł Stoch, Jacek Szczerba