Authors: Susan Schorr, Hans-Joachim Hoebler and Michael Tovar

Abstract
Stannite (Cu2FeSnS4, space group I-42m) is one of the best-known sulfide mineral, not only because of its economical importance as a tin ore, but also because of its structural and physical characteristics. The Zn-analogue kesterite, Cu2ZnSnS4, is closely related, but assigned to different space group (I-4), because of site exchange between half of the Cu and the Fe (Zn) atoms. The structures of both consist of a ccp array of sulfur atoms, with metal atoms occupying one half of the tetrahedral voids. They are topologically identical, but different in the distribution of Cu+, Zn2+ and Fe2+ among the positions (0,0,0), (0,½,¼) and (0,¼,¾). The join between Stannite and Kesterite, CuFe1-xZnxSnS4 has been the object of considerable study [1]. Structural investigations were mainly done by X-ray diffraction, notwithstanding a problem caused by the elements Cu and Zn. They are neighbours in the periodic table, thus their cations Cu+ and Zn2+ have the same number of electrons (28). The work presented here will clarify the symmetry problem along the stannite-kesterite join Cu2ZnxFe1-xSnS4. This has been the object of considerable studies described in literature before, but always done using X-ray diffraction, disregarding that Zn2+ and Cu+ have equal atomic scattering factors. To solve that problem neutron powder diffraction (experiments at Berlin Neutron Scattering Center (BENSC) with =1.79 Å) was applied, because of the different neutron scattering lengths of Zn2+, Cu+ and Fe2+. The samples used in this study were synthesized by solid state reaction from the elements. Cation site occupancies and structural parameters were determined by Rietveld analysis of the neutron diffraction data. First of all the distribution of the cations Cu+ and Zn2+ in kesterite (Cu2ZnSnS4) was revealed to be different then described in literature [2]. A clear indication for a statistic distribution of the both cations on the 2c and 2d positions was found. The way from stannite to kesterite can be described as a three stage cation re-ordering process involving Cu+, Zn2+ and Fe2+. Starting from stannite, Cu2FeSnS4 with I-42m, all substituted Zn occupies first the structural site (0,0,0) replacing Fe. In the region 0.35 < x < 0.7 the re-ordering takes place: Cu starts to occupy (0,0,0) and forces Zn on the (0,½,¼) site, at x~0.7 the (0,0,0) site is nearly random occupied by Cu, Zn and Fe. For x>0.7 Cu occupies (0,0,0) more then Zn and Fe and the fraction of Cu and Zn on (0,½,¼) becomes more and more equal. This process results at x=1 (kesterite) in a fully occupancy of (0,0,0) by Cu, the Zn and remaining Cu are disordered at (0,½,¼) and (0,¼,¾) assuming space group I-4. Our studies present impressively the power of neutron diffraction in solving the symmetry problem along the stannite-kesterite join. [1] Bonazzi et al., Can. Mineral. 41, 639-647 (2003) and references within [2] Hall et al., Can. Mineral. 16, 131 - 137 (1978).