SINGLE-CRYSTAL X-RAY AND NEUTRON-DIFFRACTION INVESTIGATIONS OF THE TEMPERATURE-DEPENDENCE OF THE STRUCTURE OF THE TC = 10-K ORGANIC SUPERCONDUCTOR KAPPA-(ET)2CU(NCS)2

The crystal structure of κ-(ET)2Cu(NCS)2 [ET or BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, C10H8S8] has been examined by single-crystal neutron and X-ray diffraction at temperatures between 298 and 15 K. Comparison of the low temperature ordered structures determined by use of X-ray and neutron diffraction with the previously reported crystallographically disordered room temperature X-ray structure indicates the avoidance of close H···H contacts as the reason for the conformational disorder of the terminal ethylene groups of the ET molecules at high temperatures. The space group is monoclinic noncentrosymmetric P21, Z = 2. Unit cell parameters at 118 K are a = 16.359(4), b = 8.418(2), c = 12.855(3) Å, β = 111.21(2)°, and V = 1650.3(7) Å3; at 15 K, a = 16.373(5), b = 8.375(3), c = 12.775(6) Å, β = 111.45(4)°, and V = 1630(1) Å3. The interlayer spacing a · sin β remains constant upon cooling from 298 to 15 K even though the a axis increases slightly in length. © 1991.