Crystal structure of picotpaulite, TlFe2S3, from Allchar, FYR Macedonia

Abstract

The crystal structure of the mineral picotpaulite, TlFe2S 3, 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 (CsFe2S 3), 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.