Crystal chemistry of natural tellurides II: Redetermination of the crystal structure of krennerite, (Au1?x Ag x ) Te2 withx?0.2

Abstract

The crystal structure redetermination of krennerite (a=16.58,b=8.849,c=4.464 A;Z=8; space group:Pma2-C2v/4) with 1063 X-ray data yielded anR-value of 5.8%. Within the structure, there are three crystallographically different noble metal positions. Two of them are statistically occupied by Au and Ag atoms (ratios of 75∶25 and 50∶50, respectively) and one position by Au atoms only. The (Au, Ag) atoms are surrounded by six Te atoms with a [2+2+2] coordination. The Au atom has a [4+2] coordination built up by six Te atoms. These three different coordination polyhedra are linked via common Te−Te edges to rugged “sheets” parallel to (010). These “sheets” are connected to a framework by four crystallographically different Te atoms—combined to one Te3 group and two Te2 dumb bells (with Te−Te<2.9 A). A fifth Te atom is only surrounded by (Au, Ag) and Au atoms. According to these different structural elements the crystal chemical formula may be written as (Au2−xAgx)Au2[Te3][Te2]2[Te];x∼0.8.