CU3SBS3 - CRYSTAL-STRUCTURE AND POLYMORPHISM

The hitherto unknown crystal structure of beta-Cu3SbS3 at room temperature could be determined from a twinned crystal. The compound crystallizes in the monoclinic system, space group P2(1)/c (No. 14), with a = 7.808(1), b = 10.233(2) and c = 13.268(2) Angstrom, beta = 90.31(1)degrees, V = 1060.1(2) Angstrom(3), Z = 8. An Extended-Huckel-Calculation shows weak bonding interactions between copper atoms which are coordinated trigonal planar. At -9 degrees C a first order phase transition occurs and the crystals disintegrate. The low-temperature modification (gamma) crystallizes in the orthorhombic system with a = 7.884(2), b = 10.219(2) and c = 6.623(2) Angstrom, V = 533.6(2) Angstrom(3) (-100 degrees C). At 121 degrees C a phase transition of higher order is observed. The high-temperature polymorph (alpha) of Cu3SbS3 is orthorhombic again. From high-temperature precession photographs the space groups Pnma (No. 62) or Pna2(1) (No. 33) can be derived. The lattice constants at 200 degrees C are a = 7.828(3), b = 10.276(4) and c = 6.604(3) Angstrom, V = 531.2(2) Angstrom(3).