NA7AU5S6 - A NOVEL THIOAURATE(I) BASED ON A BCC CHALCOGEN PACKING

Colourless transparent crystals of Na7Au5S6 were prepared by reacting a stoichiometric mixture of Na2S, gold and sulphur at 1070 K. The compound is orthorhombic, oP72, space group Pbcn (number 60) with a = 10.424(6) angstrom, b = 7.024(6) angstrom, c = 22.27(1) angstrom and Z = 4. The compound crystallizes with a new structure type, which was determined from single-crystal diffractometer data (Mo K-alpha radiation). The structure was solved by direct methods and refined to a conventional of 0.062 (R(w) = 0.059) for 685F0 with F0(2) > 4-sigma(F0(2)). The outstanding feature of Na7Au5S6 - in a partial ionic description - is the coexistence of two very different anionic species - infinite 1/infinity (AuS)- chains and isolated (AuS2)3-dumbbells - in its structure. In both anions gold is doubly coordinated by sulphur; Au-S distances are close to 2.30 angstrom; and S-Au-S bond angles vary from 170.3-degrees to 180-degrees. The Na+ ions which separate the anions are in a distorted tetrahedral sulphur coordination. The crystal structure is based on a cubic body-centred packing of chalcogen atoms; the principles of this packing are discussed.