Crystal Structure of Picotpaulite, TlFe₂S₃, from Allchar, FYR Macedonia

Tonči Balić-Žunić,^1,* Ljiljana Karanović² and Dejan Poleti³

¹ Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
² Laboratory of Crystallography, Faculty of Mining and Geology, Đušina 7, 11000 Belgrade, Serbia
³ Department of General and Inorganic Chemistry, Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia
* Corresponding author: E-mail: tonci@geo.ku.dk

Abstract
The crystal structure of the mineral picotpaulite, TlFe₂S₃, was solved and refined using single-crystal X-ray diffraction
data collected at room temperature. The symmetry is orthorhombic, space group Cmcm, with unit-cell parameters: a =
9.083(6), b = 10.754(6), c = 5.412(4) Å, V = 528.6(6) Å3, Z = 4. The structure was refined to the conventional R factor
0.0532 for 226 independent reflections with I > 2σ(I) and 21 variables. Picotpaulite is isostructural with minerals rasvumite
(KFe₂S₃) and pautovite (CsFe₂S₃), as well as with a number of synthetic compounds belonging to the CsCu₂Cl₃
structure type. The structure consists of double chains of FeS₄-tetrahedra running along [001] interconnected by TlS₁₀
coordination polyhedra, which form zig-zag chains along the same direction. The Fe–Fe distances between neighbours
along the chain direction and perpendicular to it are 2.706(2) and 2.693(6) Å, respectively, indicating strong Fe–Fe interactions.
The average oxidation state of Fe is +2.5, achieved by electron transfer over the close Fe sites. Thallium coordination
polyhedron can be described as a combination of a square antiprism adjacent to a trigonal prism. The 6s2 electrons
of Tl behave as an inert pair and the atom is situated in the centroid of its coordination.

Keywords: Picotpaulite, crystal structure, mixed valence, Fe–Fe interactions, electron delocalization, s² inert pair.