Structural and chemical variations in rathite, Pb8Pb4-x(Tl2As2)x(Ag2AS2)As16S40:: modulations of a parent structure

Members of the 4,4 sartorite homologues can be deduced from a topological parent structure with orthorhombic symmetry, space group Pmnb, a = 4.233, b = 7.918, c = 24.692 Angstrom. 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 symmetry P2(1)/c and is isostructural to synthetic Ba12Sb18.64S40 and closely related to Ba10.48Pb5.52Sb16S40 of reduced symmetry P2(1). In these structures the two-fold screw axes run parallel to b of the parent structure. Isotopological is also dufrenoysite Pb16As16S40, another 4,4 homologue of the same series, of space group symmetry P21. In dufrenoysite, however, the two-fold screw axes run parallel to c of 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 (apace group P2(1)/c; a = 8.496(1), b = 7.969(1), c = 25.122(3) Angstrom, beta = 100.704(2)degrees; R-1 = 3.67% for 2718 reflections with I > 2sigma[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)(Sigma=10.62)Ag-2.01(As19.09Sb0.83)(Sigma=19.92)S-40 derived from electron microprobe analyses matches the formula Pb10.96Ag1.90(As18.09Sb1.06)(Sigma=19.15)S-40 obtained from the structure refinement. The complex composition is caused by two independent heterovalent substitutions Ag+ + As3+ --> 2 Pb2+ and Tl+ + As3+ --> 2 Ph2+ 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 dufrenoysite Pb16As16S40. The Tl substitution in rathite appears to be optional. In addition, minor Sb3+ may replace As3+.