Paramagnetic Resonance of Vanadium (II) in Ferroelectric Potassium Ferrocyanide

Electron paramagnetic resonance of ferroelectric potassium ferrocyanide trihydrate doped with divalent vanadium has been investigated in the temperature range −180° to 20°C. Below the ferroelectric Curie temperature (T_c=−26°C) the resonance line positions may be expressed by a spin Hamiltonian for which the fine‐structure terms D and E are linearly dependent on the spontaneous polarization. Above T_c the spin Hamiltonian is independent of temperature, D is an order of magnitude smaller than at low temperatures, E=0, and the z principal axis lies in the crystal a—c plane. An anomaly in the linewidth of the $\pm \frac{3}{2}\mathrm{\to \pm }\frac{1}{2}$ satellite resonance lines is observed in the vicinity of the ferroelectric Curie temperature. With the applied magnetic field along the z principal axis, the linewidth decreases abruptly by a factor of 2 with increase in temperature through the phase transition. Dielectric constant and spontaneous polarization data are reported which agree with values previously given by Waku. The temperature dependence of the electron resonance is attributed to noncubic components of the crystal‐field potential at an iron site produced by the permanent dipole moments of aligned water molecules. Orientations of water molecules determined by NMR are used to calculate the fine‐structure tensor including both quartet and doublet T_2g excited states. A mechanism of the anomalous line broadening due to fluctuations of the spontaneous polarization is proposed. Effects of both long‐range electrostatic and short‐range hydrogen‐bond interactions in the ferroelectric transition of KFCT are qualitatively discussed.