Effect of Applied Electric Field on the Electron Paramagnetic Resonance Spectra of Doped Ferroelectric Rochelle Salt

A linear shift of the epr lines of ${\mathrm{Cu}}^{2+}$ -doped Rochelle salt by an applied electric field has been observed in the high-temperature paraelectric phase. The shifts become asymmetric with respect to the polarity of the applied field in the ferroelectric phase and disappear at low temperatures in the antiferroelectric phase, demonstrating that it is only the instability of the crystal lattice connected with the ferroelectric transitions which makes the electric-field effect easily observable.