Structure of potassium cyanide at low temperature and high pressure determined by neutron diffraction.

Neutron time-of-flight powder-diffraction spectra have been obtained in potassium cyanide (KCN) at temperatures between 25 and 200 K and at pressures between 35 and 366 MPa. These data sample Ave difFerent phases of KCN. In phase C, which exists at 366 MPa and below 170 K, the structure is determined to be paraelectric with the monoclinic space-group symmetry C2/c. This is in disagreement with a previous study which determined the structure to be ferroelectric with space-group symmetry Cc. In phase D, which exists at 366 MPa and below 120 K, the structure is determined to be paraelectric with the monoclinic space-group symmetry C2/m. The cyanide ions lie in mirror planes. In both phases C and D, the cyanide ions are ordered with respect to the direction of their bonds but disordered with respect to interchange of C and N. Also in these phases, line broadening due to an inhomogeneous shear strain was observed. A mathematical treatment of this phenomenon is given.