On the first incommensurate ordering of NH4Cu7-xS4

The first incommensurate ordering of NH4Cu7-xS4 (x = 0.16) has been investi gated using X-ray single crystal diffraction at 174 K. The modulated struct ure is described with monoclinic symmetry with the cell parameters alpha = 10.272 (Angstrom), b = 10.272 (Angstrom), c = 7.694 (Angstrom) and beta = 9 0.0 degrees. It belongs to the four-dimensional space group P:P2/n:-11 {con ventional setting P:P2/b:-11 (no. 13.1)}. The incommensurate modulation to the lattice (q approximate to 0.39, 0, 0) appears to be caused by the sub-s toichiometry of copper. A DSC-study of the isostructural TiCu7-xS4 relates the thermal behaviour of the two thiocuprates to changes in the copper cont ent. A tentative 'phase stability diagram' for such compounds is presented. (C) 1999 Elsevier Science S.A. All rights reserved.