STRUCTURE OF POTASSIUM CYANIDE AT LOW-TEMPERATURE AND HIGH-PRESSURE DETERMINED BY NEUTRON-DIFFRACTION

Neutron time of-flight powder-diffraction spectra have been obtained i n potassium cyanide (KCN) at temperatures between 25 and 200 K and at pressures between 35 and 366 MPa. These data sample five different pha ses of KCN. In phase C, which exists at 366 MPa and below 170 K, the s tructure is determined to be paraelectric with the monoclinic space-gr oup symmetry C2/c. This is in disagreement with a previous study which determined the structure to be ferroelectric with space-group symmetr y Cc. In phase D, which exists at 366 MPa and below 120 K, the structu re is determined to be paraelectric with the monoclinic space-group sy mmetry C2/m. The cyanide ions lie in mirror planes. In both phases C a nd D, the cyanide ions are ordered with respect to the direction of th eir 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.