Crystallographic analysis of structures associated with a number of Tl- and Ag-containing sulfides: the composition–symmetry correlation

Abstract

A crystallographic analysis is conducted of the structures of orthorhombic mineral sicherite TlAg₂(As,Sb)₃S₆, monoclinic synthetic sulfide Tl₃Ag₃Sb₂S₆, and triclinic mineral raberite Tl₅Ag₄As₆SbS₁₅. In the first two structures, the large and heavy Tl⁺ cation forms, together with the other cations, ordered “skeletal” frameworks with F and I cation sublattices that are close to cubic ones. In the structure of raberite, the Tl and Ag cations undergo, together with the sulfur anions, two-dimensional ordering by a zone of closely packed crystallographic planes, which generate a pseudohexagonal symmetry. The deviations from the 1 cation/anion stoichiometry are compensated: in the second structure, by a local consolidation of cations (to a distance Tl–Ag = 2.96 Å) and, in the third structure, through the formation of a dumbbell pair As–Ag (2.68 Å), which occupies one position in the sublattice.