X-ray diffraction evidence for a monoclinic form of stibnite, [Sb.sub.2][S.sub.3], below 290 K

The nature of the symmetry changes associated with phase transitions in stibnite, [Sb.sub.2][S.sub.3], has been investigated by powder and single crystal X-ray diffraction. Above 420 K stibnite (I) is centrosymmetric Pbnm. Stibnite (II) is stable between 290 and 420 K and is acentric [P2.sub.1]nm. Below 290 K stibnite (III) is stable and systematic absences indicates that this phase is monoclinic with space group [P2.sub.1] or Pm, or triclinic P1, but the cell dimensions remain geometrically orthorhombic. These findings are in agreement with variations in physical and electrical properties reported in the literature.