Na23Magnetic-Resonance Study of the Ferroelectric Transition in Rochelle Salt

The angular dependence of the second-order quadrupole shifts of the ${\mathrm{Na}}^{23}$ magnetic resonance has been measured in the two paraelectric phases of both deuterated and normal Rochelle salt, as well as in the intermediate ferroelectric phase, where an additional splitting of the ${\mathrm{Na}}^{23}$ lines has been found. The electric quadrupole coupling constants, asymmetry parameters, and orientations of the principal axes of the electric-field-gradient (EFG) tensors at the sodium sites have been determined. The results of a point-charge-model calculation show that the changes in the EFG tensors on going from the low-temperature paraelectric phase to the ferroelectric phase can be explained by a displacement of two out of the four O(5)-H hydrogens and O(8) water molecules in the unit cell, the principal component of the displacement vector pointing in the direction of the ferroelectric axis.