Die Kristallstruktur der Tieftemperaturmodifikation von Ag₈GeS₆

Abstract

The low temperature form of Ag₈GeS₆ (synthetic argyrodite) is orthorhombic, space group Pna2₁, witha=15.149 (1),b=7.476 (2),c=10.589 (1), andZ=4. The crystal structure has been determined from 2-circle diffractometer data by means of direct methods and refined toR=0.081 for 3431 intensities. The structure consists of slightly distorted isolated GeS₄ tetrahedra (mean Ge−S bond length 2.212 Å) and two further sulphur atoms without bonds to the germanium atoms. The GeS₄ tetrahedra and the S atoms are connected by the Ag atoms to form a three-dimensional framework. Three types of Ag coordination by S atoms are encountered. Three Ag positions have a strongly distorted tetrahedral environment, four Ag positions an approximately planar threefold coordination, while one Ag atom is almost linearly coordinated by two S atoms. The Ag−S distances are 2.56–2.94 Å, 2.49–2.76 Å, and 2.42–2.44 Å, resp. All Ag atoms have at least one near Ag neighbour between 2.93 and 3.11 Å.