DETERMINATION OF THE CRYSTAL STRUCTURE OF SKINNERITE, P21/c-Cu3SbS3, FROM POWDER DATA

EMIL MAKOVICKY and TON I BALI - UNI
Department of Mineralogy, Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark

Abstract

The crystal structure of the room-temperature polymorph of skinnerite, Cu3SbS3, was determined by the Rietveld method from Guinier diffractometer X-ray powder data. The space group is P21/c; the cell parameters are a 7.814(1), b 10.242(1), c 13.273(1) Å, 90.29(1) , with Z = 8. The sulfur framework can be described as a (deformed) (112)hcp twinned hcp array. Sb has trigonal pyramidal coordination in the bases of trigonal coordination prisms created by the unit-cell twinning of the S array. The is between 2.44 and 2.47 Å in length. Most Cu sites have a triangular coordination, with distances in the range 2.26-2.31 Å. Cu4 is split into two irregularly tetrahedral, [3 + 1] coordinations, with in the range 2.40-2.46 Å. Cu3 and Cu4 are in triangular faces common to two tetrahedral voids of the S array, and the remaining Cu atoms are in such faces common to tetrahedral and octahedral voids. P21/c-skinnerite is a low-temperature polymorph of the ionic conductor Cu3SbS3. Differences in the distribution of Cu between P21/c-skinnerite and P212121-wittichenite are caused by different sizes of Sb and Bi coordination polyhedra.

Keywords: skinnerite, room-temperature polymorph, crystal structure, Rietveld refinement.