Crystal Structures of the Room- and Low-Temperature Phases of Monoclinic Potassium Ferricyanide

Abstract

Crystals of K₃Fe(CN)₆ undergo a second-order phase transition accompanied by a soft mode at about 130 K. The crystal structures of the room- and low-temperature phases have been determined by X-ray diffraction. Crystals of the room-temperature phase are monoclinic with space group P2₁⁄c, a=7.058(1), b=10.426(1), c=8.401(1) Å, β=107.31(1)° and Z=2 at 293 K. Those of the low-temperature phase are monoclinic with space group P2₁⁄n, a=14.021(2), b=10.401(1), c=8.339(2) Å, β=107.21(1)° and Z=4 at 100 K. Final R values were 0.029 for 1591 observed reflections and 0.027 for 2642 observed reflections for the room- and low-temperature phases respectively. With a transition from room- to low-temperature phase, [Fe(CN)₆]³⁻ anions displace by about 0.1 Å along the b axis. A soft mode is considered to be a translational lattice mode of [Fe(CN)₆]³⁻ anions which has a wave vector a^*⁄2 in the room-temperature phase.