Potassium-cyanide-phase-I-unit-cell-3D-SF

Space-filling model of the unit cell of phase I (rock-salt structure) of potassium cyanide, KCN.

Potassium ions, K⁺, are represented by purple spheres, and cyanide ions, CN⁻, exbihit dynamic rotational disorder and are represented by magenta spheres. The lattice parameter a = 6.4802 Å, the temperature is 200 K and the pressure is 54 MPa.

X-ray crystallographic data from H. T. Stokes, D. L. Decker, H. M. Nelson, J. D. Jorgensen (1993). "Structure of potassium cyanide at low temperature and high pressure determined by neutron diffraction". Phys. Rev. B 47 (17): 11082-11092. DOI:10.1103/PhysRevB.47.11082..

"The CN⁻ ions undergo rapid reorientations centered at the Wyckoff 4a position (0,0,0). The time-averaged symmetry of the CN⁻ ions about that point is cubic (point group m-3m). The rotation is not free, but is hindered. The potential energy of the CN⁻ ion is lower for the <111> directions of orientation, and the CN⁻ ion spends more time near those minima."

Model constructed in CrystalMaker 8.1.