On the first incommensurate ordering of NH₄Cu_7-xS₄

The first incommensurate ordering of NH₄Cu_7-xS₄ (x = 0.16) has been investigated using X-ray single crystal diffraction at 174 K. The modulated structure is described with monoclinic symmetry with the cell parameters a = 10.272 (angstrom), b = 10.272 (angstrom), c = 7.694 (angstrom) and β=90.0°. It belongs to the four-dimensional space group P:P2/n:-11 {conventional setting P:P2/b:-11 (no. 13.1)}. The incommensurate modulation to the lattice (q approx. 0.39, 0, 0) appears to be caused by the sub-stoichiometry of copper. A DSC-study of the isostructural TlCu_7-xS₄ relates the thermal behaviour of the two thiocuprates to changes in the copper content. A tentative 'phase stability diagram' for such compounds is presented.