Hyperfine interactions in szomolnokite (FeSO 4 H 2 O) Antoine Van Alboom a,b, * , Valdirene G. De Resende b , Eddy De Grave b , J. Alexandra M. Gómez b a Faculty of Applied Engineering Sciences, University College Gent, Schoonmeersstraat 52, B-9000, Gent, Belgium b Department of Subatomic and Radiation Physics, University of Ghent, Proeftuinstraat 86, B-9000, Gent, Belgium article info Article history: Received 5 October 2008 Accepted 22 October 2008 Available online 6 November 2008 Keywords: Szomolnokite 57 Fe-Mössbauer External magnetic field EFG calculation Crystal field abstract The temperature dependencies of the ferrous Mössbauer parameters for synthetic szomolnokite are determined in the range between 4 and 450 K. A magnetic order-disorder transition is observed at 29.6 ± 0.5 K. The Mössbauer spectra in the paramagnetic region are well fitted by the superposition of a Fe 2+ distributed quadrupole component and a small ferric doublet component. The spectra in the mag- netic region are typically for Fe 2+ , showing up to eight absorption lines. They are analyzed from diagonal- ization of the full nuclear-interaction hamiltonian. The saturation value of the hyperfine field is 32.5 T. For temperatures below 29 K, the adjusted value for the asymmetry parameter of the EFG is g  0.63 and for the quadrupole splitting DE Q  3.08 mm/s. These values are in line with those measured at tem- peratures in the paramagnetic regime. The temperature dependence of the quadrupole splitting is inter- preted within the ferrous 5 D orbital energy level scheme by a crystal field calculation based on the point symmetry of the Fe 2+ site in szomolnokite. It is found that the high asymmetry of the EFG is caused by the presence of the H 2 O groups in the structure. The variation of the hyperfine field with temperature is inter- preted within the molecular field approximation, however, taking exchange magnetostriction into account. From the temperature dependence of the isomer shift the characteristic Mössbauer temperature H M is determined to be 425 K, from which the Mössbauer fraction at room temperature is estimated to be 0.79. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Szomolnokite (FeSO 4 H 2 O) is a hydrous ferrous sulfate belong- ing to the kieserite group of sulfate minerals. In nature it can be found in the oxidation zones of sulfide deposits. Since the recent Mars mission by the Mars Exploration Rovers [1,2] sulfates in gen- eral became of renewed interest because they are likely to be pres- ent in Martian soil and because the presence of hydrous sulfates especially provides evidence for the presence or pre-existence of water on Mars [3,4]. The crystal structure of szomolnokite (Fig. 1, after Papike et al. [5]) consists of corner sharing FeO 6 octahedra forming chains parallel to the [0 0 1] crystallographic axis. These chains are cross-linked into a framework of SO 4 tetrahedra and the structure is strengthened by hydrogen bonds. It has a mono- clinic crystallographic structure (C2/c). Cell parameters, as deter- mined by single-phase Rietveld refinements of X-ray powder diffraction data [6,7], are a = 7.078(3), b = 7.549(3) and c = 7.773(3) Å, and b = 118.65°(2). The Fe atom has point symmetry  1 and is coordinated to six oxygen atoms forming an elongated octahedron. Mössbauer data reported in the literature about szomolnokite are very limited. Lane [1] shows a Mössbauer spectrum (MS) at 220 K without indication of the numeric values of the Mössbauer parameters. Bandyopadhyay [8] presented a MS of a coal sample at room temperature (RT), containing a subspectrum with center shift (d) of 1.23 mm/s and quadrupole splitting (DE Q ) of 2.74 mm/s and assigned this subspectrum to a szomolnokite phase. Eymery et al. [9] studied a mechano-chemical transformation of iron in pyrite and reported d = 1.24 mm/s and DE Q = 2.72 mm/s for a szomolnokite phase present in one of their samples. This paper concerns a thorough investigation of a synthetic szomolnokite by Mössbauer spectroscopy, giving the temperature (T) dependencies of the various ferrous Mössbauer parameters. The obtained hyperfine data for both the paramagnetic and the magnetically ordered state are interpreted on the basis of known theoretical models. 2. Experimental The szomolnokite sample was synthesized by heating commer- cial heptahydrate sulfate [FeSO 4 7H 2 O] at 200 °C during 2 h in an inert N 2 atmosphere in order to avoid oxidation of the ferrous ions. The X-ray diffraction (XRD) pattern of the obtained product is reproduced in Fig. 2 and confirms that it concerns single-phase szomolnokite. 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.10.049 * Corresponding author. Address: Faculty of Applied Engineering Sciences, University College Gent, Schoonmeersstraat 52, B-9000, Gent, Belgium. Tel.: +32 9 24 24 229. E-mail address: toon.vanalboom@hogent.be (A.V. Alboom). Journal of Molecular Structure 924–926 (2009) 448–456 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc