The incommensurately modulated crystal structure of roshchinite, Cu0.09Ag1.04Pb0.65Sb2.82As0.37S6.08

Abstract

Abstract The Pb–Ag–Sb sulfide roshchinite, Cu0.09Ag1.04Pb0.65Sb2.82As0.37S6.08, 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(0σ 20)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 Robs (wR) equal to 7.27 (8.07)% using satellites up to second order; Robs 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.