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 fr om a 12-fold twinned crystal. The symmetry operations and the unit cell of the pseudo-cubic expression rue (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 Angstrom, c = 9.870 Angstrom, alpha = beta = 90 degrees, gamma = 90.64 degrees, whose standard settings are Pmn2(1); A = 7.045, B = 6.966, C = 9.870 Angstrom. 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 mino r 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 Cu8G eS6, Cu ions are expected to distribute statistically over many sites in th e framework. On cooling through the phase-transition temperature, the cryst al can be converted into a multiple twin of phase II. Cu ions are transferr ed to five kinds of independent sites from the statistical distribution sta te. (C) 2000 Elsevier Science B.V. All rights reserved.