Mixed crystals in the system Cu2MnGexSn1−xS4: Phase analytical investigations and inspection of tetrahedra volumes

Cu2MnGeS4 crystallizes orthorhombic in a wurtzite superstructure type while Cu2MnSnS4 crystallizes in a tetragonal sphalerite superstructure type. Lattice constants and thermal analyses of the solid solution series Cu2MnGexSn1−xS4 are presented. A two-phase region is found from Cu2MnGe0.3Sn0.7S4 to Cu2MnGe0.5Sn0.5S4. The cell volume of the mixed crystals increases with increasing Sn content. The melting points increase smoothly with increasing Ge content to x=0.5x=0.5 and then steeply for higher Ge contents. The single crystal X-ray structure analysis of Cu2MnGe0.55Sn0.45S4 is presented. The refinement converges to R=0.0270R=0.0270 and wR2=0.0586wR2=0.0586, Z is 2. The volumes of the tetrahedra [MS4] (M=Cu, Mn, Ge, Sn) are calculated. From these volumes the differences in size of the tetrahedra are derived and compared with the corresponding differences in the end members of the solid solution series. It turns out that the resulting structure type in these materials depends on the volume differences of the constituting tetrahedra [MS4].

The system Cu2MnGeS4–Cu2MnSnS4 is inspected for the formation of mixed crystals at a temperature of 800 °C. The wurtzstannite structure type of Cu2MnGeS4 exists to 60% germanium content. The stannite structure type dominates from 20% germanium content to the pure Sn compound. Germanium and tin occupy a 2a position and are statistically disordered. Tetrahedra volumes [MS4] of Cu2MnGe0.55Sn0.45S4 are compared with those of the end members and are used as a measure for the preference of the different structure type.Figure optionsDownload as PowerPoint slide