The crystal structure of Tl2.36Sb5.98As4.59S17, the lead-free endmember of the chabournéite homeotypic group

Abstract

ABSTRACT The crystal structure of Tl2.36Sb5.98As4.59S17, the lead-free endmember of the chabournéite homeotypic group, from the Tl-As-Sb-rich gold deposit at Vorontsovskoye (the Urals, Russia) was determined and refined to R(obs) 0.099 for 9340 unique observed X-ray reflections. The triclinic unit-cell parameters determined from single-crystal data are as follows: a = 8.63253(19) Å, b = 16.3055(7) Å, c = 21.8196(8) Å, α = 75.094 (3)°, β = 83.631(2)°, γ = 89.303 (2)°, V = 2949.18(18) Å3 (Z = 4), space group . The crystal structure is composed of (001) slabs based on PbS and SnS archetypes, arranged in regular alternation. All Sb(As) coordination polyhedra are (Sb,As)S3+2+(1 or 2) coordination pyramids, in the majority of cases with a mixed Sb-As occupancy in both slab types. Bond-length distributions were studied in detail. The zig-zag boundary between the slabs is composed of a repeating sequence of [100] Tl-Tl, Sb-Sb (1/3 substituted by As), Tl-Tl, and Tl-Sb columns. Thallium forms tricapped trigonal coordination prisms and (Sb,As) forms bicapped prisms. Differences compared to two related structures—parapierrotite and tsygankoite—are specified. Twinning of chabournéite is connected with the (imperfect) order-disorder character of the structure, which is connected with the configurations observed on slab boundaries. The structure refinement of the lead-free Tl-(Sb,As) chabournéite endmember presented in this paper is the best starting point for a restudy of all complexities of the chabournéite homeotypic group.