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E. Makovicky, B. Stöger, D. Topa:
"The incommensurately modulated crystal structure of roshchinite, Cu_0.09Ag_1.04Pb_0.65Sb_2.82As_0.37S_6.08";
Zeitschrift für Kristallographie - Crystalline Materials, 233 (2018), 3-4; 255 - 267.

The Pb-Ag-Sb sulfide roshchinite, Cu_0.09Ag_1.04Pb_0.65Sb_2.82As_0.37S_6.08, is a lillianite homologue N=4 with a complex incommensurate superstructure in the 8.46 Å direction of the orthorhombic crystal lattice with a 19.0804(1) Å, b 8.4591(2) Å and c 12.9451(3) Å, superspace group Pbcn(0σ20)00s, q=0.41458(3)b*. A structure solution and refinement in (3+1) superspace, based on 10,019 observed reflections and 437 refined parameters was terminated at R_obs (wR) equal to 7.27 (8.07)% using satellites up to second order; R_obs is 4.82 for main reflections only. The Pb atoms in trigonal coordination prisms on planes of unit-cell twinning are semiperiodically replaced by antimony. The marginal columns of (311) PbS slabs of the Sb-Ag based structure which is based on PbS-like topology contain Sb, Ag(Cu) and mixed Ag/Sb sites in a complicated sequence. Central portions of the slabs are occupied by Sb-S crankshaft chains, best exposed on the (100)PbS planes, which run diagonally across the slabs. In these planes, in their majority the chains display Sb₃S₄ form and two opposing orientations, zig-zagging along the [010] direction. Every six chains, a parallel configuration of two chains occurs, but occasionally this interval is reduced to five chains. This, together with related compositional changes in the Pb- and Ag-Sb column, explains the one-dimensionally incommensurate character of roshchinite. Modestly elevated contents of As replacing Sb are the probable reason of modulation and non-commensurability in roshchinite.

incommensurate structure; interface modulation; lillianite homologue; Pb-Ag-Sb-As sulfide; roshchinite; (3+1) superspace refinement

http://dx.doi.org/10.1515/zkri-2017-2126