The crystal structure of ferdowsiite Ag₈Sb₄(As,Sb)₄S₁₆ and its relations to other ABX₂ (A=Ag; B=As,Sb,Bi; X=S,Se) structures

Abstract

Ferdowsiite is monoclinic, a=8.677(2) Å, b=5.799(1) Å, c=13.839(3) Å, β=96.175(4)°, unit cell volume 692.3 ų. Space group P2₁/n. Refinement from single-crystal X-ray diffraction data reached R₁=0.028 for 626 F_o>4σ(F_o) and 0.032 for all 716 F_o used. Value of wR₂ is 0.065. The crystal structure of ferdowsiite, approximately Ag₈Sb₄(As,Sb)₄S₁₆, contains four distinct cation and four different anion sites in the asymmetric unit, all in general positions. Besides two Ag sites and one Sb site, the crystal structure contains one mixed As-Sb coordination polyhedron (0.63 As and 0.37 Sb in the site). The Sb1 site has three short Sb-S bonds 2.503–2.645 Å. As and Sb in the mixed site were refined separately, with isotropic displacement coefficients. As has typical bond length values of 2.248–2.354 Å whereas Sb has 2.443–2.392 Å, i.e., the observed ligand positions are visibly influenced by the predominant arsenic. The crystal structure of ferdowsiite is a superstructure of a PbS like motif. The {100} planes of the PbS-like substructure are the (105̅), (301) and (010) planes in terms of the ferdowsiite lattice. The structure contains zig-zag chains of Sb1 connected via short Sb-S bonds and flanked by (Sb,As)S₃ groups and Ag polyhedra. Groups of four SbS₅-and (Sb,As)S₅ coordination pyramids form an interconnected network with interspaces which accommodate both the lone electron pairs and the tetrahedrally coordinated Ag situated in coordination octahedra. Partial lead substitution takes place in the coordination polyhedra of Sb1 and Ag2. In the paper, the structure is compared with the other Ag(As,Sb,Bi)(S,Se)₂ structures. The configurationally closest PbS-based homeotypic structure, however, is diaphorite, which is an ordered superstructure of the ferdowsiite arrangement, with a substantial presence of Pb, and without As.