Na 7Au 5S 6 — A novel thioaurate(I) based on a b.c.c. chalcogen packing

Abstract

Colourless transparent crystals of Na 7Au 5S 6 were prepared by reacting a stoichiometric mixture of Na 2S, gold and sulphur at 1070 K. The compound is orthorhombic, oP72, space group Pbcn (number 60) with a=10.424(6) A ̊ , b=7.024(6) A ̊ , c=22.27(1) A ̊ and Z=4. The compound crystallizes with a new structure type, which was determined from single-crystal diffractometer data (Mo Kα radiation). The structure was solved by direct methods and refined to a conventional R of 0.062 ( R w =0.059) for 685 F o with F o 2>4 σ( F o 2). The outstanding feature of Na 7Au 5S 6 — in a partial ionic description — is the coexistence of two very different anionic species — infinite ∞ 1(AuS) − chains and isolated (AuS 2) 3− dumbbells — in its structure. In both anions gold is doubly coordinated by sulphur; Au-S distances are close to 2.30 Å; and S-Au-S bond angles vary from 170.3° to 180°. 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.