Characterization of mixed crystals in the system Cu2Mn x-Co1-xGeS4 and investigations of the tetrahedra volumes

Abstract

In a series of investigations on normal tetrahedral compounds we present mixed crystals in the system Cu2MnxCo 1-xGeS4 (0 < x < 1) and an inspection of their tetrahedra volumes. Cu2CoGeS4 crystallizes tetragonal in a stannite type structure, Cu2MnGeS4 crystallizes orthorhombic in the wurtzstannite structure type. The crystal structures of Cu2CoGeS4 and Cu2Mn0.68Co 0.32GeS4 were refined from single crystal diffraction data. The refinement of Cu2CoGeS4 converged to R = 0.0547 and wR2 = 0.0847 for 299 unique reflections. The refinement of Cu 2Mn0.68Co0.32GeS4 converged to R = 0.0481 and wR2 = 0.0877 for 1556 unique reflections. From these data the tetrahedra volumes of the end members and of Cu2Mn 0.68Co0.32GeS4 are calculated. In Cu 2CoGeS4 tetrahedra [MS4] are similar in size. In contrast, the differences of the volumes of the polyhedra [MS4] in the orthorhombic wurtzite superstructure type compounds Cu 2MnGeS4 and Cu2Mn0.68Co 0.32GeS4 are significant (M = Cu, Mn, (Mn 0.68Co0.32), Co, Ge). From x = 0 to x = 0.5 the tetragonal structure type dominates while from x = 0.7 to the Cu2MnGeS 4 end member the products crystallize in the orthorhombic structure type. Melting points of the mixed crystals decrease linearly with increasing manganese content. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.