Multiple-twinned crystal of the room-temperature phase of Cu8GeS6

The structure of the orthorhombic room-temperature phase of Cu8GeS6 (phase II) has been refined successfully on the basis of X-ray diffraction data from a 12-fold twinned crystal. The symmetry operations and the unit cell of the pseudo-cubic expression are (0,0,0; 1/2,1/2,0)+ x,y,z; y,x,z; 1/4−x,3/4−y,1/2+z; 3/4−y,1/4−x,1/2+z; a=b=9.907 Å, c=9.870 Å, α=β=90°, γ=90.64°, whose standard settings are Pmn21; A=7.045, B=6.966, C=9.870 Å. Among the twin domains 1–6, the argyrodite-type GeS6 frameworks can be superposed to each other in principle, and only Cu–Cu network directions differ. The minor twin domains 7–12 are respectively related to the major domains 1–6 by a two-fold rotation on [111]. In the high-temperature phase (phase I) of Cu8GeS6, Cu ions are expected to distribute statistically over many sites in the framework. On cooling through the phase-transition temperature, the crystal can be converted into a multiple twin of phase II. Cu ions are transferred to five kinds of independent sites from the statistical distribution state.