CRYSTAL GROWTH OF ARGYRODITE Ag8GeS6

Abstract

The mineral Ag8GeS6 is the first compound of a wide class of tetrahedrally close packed phases united by the common name argyrodites. Ternary argyrodites are complex chalcogenides formed on the basis of multi- and single-charged cations. Structural polyhedrons are tetrahedra formed on the basis of a multicharged cation and chalcogens, and the single-charged cations occupy the formed voids. Argyrodites belong to promising solid-state conductors and thermoelectric materials. This work is aimed to develop close to optimal conditions for single crystals growth of Ag8GeS6 by the method of directional crystallization from the melt. The initial Ag8GeS6 alloy was obtained using high purity elementary components by a two-stage single-temperature method. The congruent nature of the melting of Ag8GeS6 (Tmelt = 958°C) and the passage of the structural phase transition Pna21 → F-43m at a temperature of 227 ° C was confirmed by the DTA method. The Ag8GeS6 single crystal was grown from the melt by directional crystallization method. The growth regime of Ag8GeS6 was selected taking into account the nature of the thermal behavior of the compound based on DTA data. The grown single crystal of Ag8GeS6 is ~ 30 mm long and 12 mm in diameter and has a medium degree of cleavage. According to the results of XRD analysis, it was established that the obtained single crystalline sample of Ag8GeS6 is single-phase and crystallizes by low-temperature modification SG Pna21 with following lattice parameters: a = 15.147 Å, b = 7.469 Å, c = 10.584 Å.