Toward the crystal structure of nagyagite, [Pb(Pb,Sb)S2][(Au,Te)]

Abstract

Synthetic nagyagite was grown from a melt as part of a search for materials with high-temperature superconductivity. Electron microprobe analyses of synthetic nagyagite and of nagyagite from the type locality Nagyag, Transylvania (now Sacarimb, Romania) agree with data from literature. The crystal chemical formula [Pb(Pb,Sb)S2][(Au,Te)] was derived from crystal structure investigations. Nagyagite is monoclinic pseudotetragonal. The average crystal structure was determined from both synthetic and natural samples and was refined from the synthetic material to R = 0.045 for 657 single-crystal X-ray data: space group P21/m, a = 4.220(1) Å, b = 4.176(1) Å, c = 15.119(3) Å, β = 95.42(3)°, and Z = 2. Nagyagite features a pronounced layer structure: slices of a two slabs thick SnS-archetype with formula Pb(Pb,Sn)S2 parallel to (001) have a thickness of 9.15 Å. Te and Au form a planar pseudo-square net that is sandwiched between the SnS-archetype layers; it is assumed that planar Au([4Te])Te4 configurations are edge connected to chains and that Te atoms are in a zigzag arrangement. Ordering within the SnS-archetype and gold-tellurium layers, intense twinning and/or stacking variants are responsible for the often observed superstructure reflections. For buckhornite, [(Pb2Bi)(Σ3)S3][(AuTe2)(Σ3)], a structure model is proposed considering a homologous series with nagyagite, [(Pb3(Pb,Sb)3)(Σ6)S6][(Au,Te)3].