Na23Quadrupole Interaction in Ferroelectric Rochelle Salt

The first-order electric quadrupole splitting of the ${\mathrm{Na}}^{23}$ nuclear magnetic resonance has been studied in single crystals of undeuterated Rochelle salt. At 30°C, in the upper paraelectric phase, the quadrupole coupling constants and asymmetry parameters for all four ${\mathrm{Na}}^{23}$ sites were found to be the same: $\frac{e^2\mathrm{Qq}}{h}=1.313\mathrm{}$ Mc/sec and $\eta =0.809\mathrm{}$. At 0°C in the ferroelectric phase, two coupling constants were measured, corresponding to the lower crystal symmetry. The values are $\frac{e^2\mathrm{Qq}}{h}=1.285\mathrm{}$ Mc/sec with $\eta =0.872\mathrm{}$, and $\frac{e^2\mathrm{Qq}}{h}=1.407\mathrm{}$ Mc/sec with $\eta =0.675\mathrm{}$. These results were analyzed according to the model of Blinc, Petkovsek, and Zupancic in which the changes are due to displacements of one hydroxyl ion and one water molecule. The data fit the model well and lend support to their view of the structural changes that take place when Rochelle salt becomes ferroelectric.