A neutron diffraction study of ferroelectric KFCT, K₄Fe(CN)₆.3D₂O, above the Curie temperature

A neutron diffraction study of deuterated potassium ferrocyanide trihydrate (KFCT) has been made above the ferroelectric Curie temperature. The monoclinic form, space group A2/a (No. 15), has a = 9.42, b = 17.01, c = 9.38 Å, and β= 90.03° at 20°C. Since all of the large crystals of KFCT were twinned by a 90° rotation about the b axis, data from both overlapped and nonoverlapped reflections were used in the determination of the structure. Refinement by a modified least-squares method that accounted for the overlapped data led to a weighted R value on F2 (3678 reflections) of 4.8%. The corresponding R on F is 4.9%. The disordered water molecules are of two types: one is disordered into two positions related by a twofold axis parallel to b: the other is disordered into two positions that are related by an approximate 90° rotation in the plane of the molecule. Both types of water molecules show O–D ⋯O or O–D⋯N bonding. The Fe(CN)4−6 octahedra are regular and the potassium ions are loosely surrounded by O and N atoms. This analysis has elucidated the role of the water molecules in the ferroelectric switching mechanism. The [{ 1}01] ferroelectric axis in this crystal can be explained on the basis of the dipole moment and orientation of the water molecule dipoles.