The crystal structure of gustavite, PbAgBi_3S_6 Analysis of twinning and polytypism using the OD approach

摘要

The crystal structure of gustavite Pb_(1.06)Ag _(0.95)Bi_(2.80)Sb_(0.20)S_(5.99) from Rotgülden, Lungau, Salzburg Land, Austria, was refined to R_1 = 2.76 using single-crystal X-ray diffraction data. Refined unit-cell parameters are a 7.0567(14), b 19.6905(39), c 8.2219(16) A°, β 106.961(3)°, unit-cell volume 1092.7(2) A°~3 and space group is P2 _1/c. The structure contains a bicapped trigonal prismatic Pb site, a 001 string of Ag and Bi1 octahedral sites and a similar string of Bi _2 and Bi_3 coordination octahedra. Sb is concentrated in the Bi_3 site whereas Bi_1 and Ag contain a fraction (0.1-0.14 atom) of the opposite species; the latter is explained by a presence of twin lamellae or antiphase domains in the crystal. In accordance with its frequently observed twinning, gustavite is an OD (order-disorder) structure that can be described by zig-zag unit layers with layer symmetry P(2/n) 2 _1/c 2/m and the interlayer symmetry elements {(2_2/n _(1,1/2)) 2_1/n_(1/2,2)2_(1/2)/a _2}. An alternative OD description uses planar unit layers with the layer symmetry P(2/b) 2_1/c 2_1/m and the interlayer elements 2_2/n_(1,-1/2) 2_1/n_(1/2,2) 2 _(1/2)/a_2. The former description adheres closely to the details of coordination of bismuth observed in the structure whereas the latter description is based on a more abstract structure model.