Structural and chemical variations in rathite, Pb8Pb4—x(Tl2As2)x(Ag2As2)As16S40: modulations of a parent structure

Abstract

AbstractMembers of the 4,4 sartorite homologues can be deduced from a topological parent structure with orthorhombic symmetry, space groupPmnb,a= 4.233,b= 7.918,c= 24.692 Å. The observed modulations in 4,4 sartorite homologues are associated with symmetry lowering of the parent structure and superstructure formation. The Pb, Tl, Ag, As, Sb sulfosalt rathite is a 4,4 member of the sartorite homologous series with space group symmetryP21/cand is isostructural to synthetic Ba12Sb18.64S40and closely related to Ba10.48Pb5.52Sb16S40of reduced symmetryP21. In these structures the two-fold screw axes run parallel tobof the parent structure. Isotopological is also dufrénoysite Pb16As16S40, another 4,4 homologue of the same series, of space group symmetryP21. In dufrénoysite, however, the two-fold screw axes run parallel tocof the parent structure. To elucidate the modulation of the parent structure, rathite has been structurally and chemically reinvestigated emphasising cation order/disorder relations. Single crystal X-ray structure refinement (space groupP21/c;a= 8.496(1),b= 7.969(1),c= 25.122(3) Å,β= 100.704(2)°;R1= 3.67% for 2718 reflections withI> 2σ[I]) yielded three split cation positions indicating heterovalent substitutions. Considering that isoelectronic Pb2+and Tl+cannot be distinguished in our diffraction experiment the chemical composition (Pb9.25Tl1.37)Σ=10.62Ag2.01(As19.09Sb0.83)Σ=19.92S40derived from electron microprobe analyses matches the formula Pb10.96Ag1.90(As18.09Sb1.06)Σ=19.15S40obtained from the structure refinement. The complex composition is caused by two independent heterovalent substitutions Ag++ As3+→ 2 Pb2+and Tl++ As3+→ 2 Pb2+leading to the simplified rathite formula Pb8Pb4–x(Tl2As2)x(Ag2As2)As16S40. The substitution involving Ag seems to be essential for the formation of rathite and distinguishes rathite from dufrénoysite Pb16As16S40. The Tl substitution in rathite appears to be optional. In addition, minor Sb3+may replace As3+.