${\mathrm{Rb}}^{87}$ and ${\mathrm{As}}^{75}$ Quadrupolar Coupling in Ferroelectric Rb${\mathrm{H}}_2$P${\mathrm{O}}_4$ and Cs${\mathrm{H}}_2$As${\mathrm{O}}_4$

The quadrupole-induced shifts of the ${\mathrm{Rb}}^{87}$ and ${\mathrm{As}}^{75}$ ½ ↔ -½ nuclear-magnetic-resonance transitions have been used to study the temperature variation of the electric field gradients at the ionic sites on approaching the ferroelectric Curie points. The shifts can only be explained in terms of collective atomic fluctuations in the paraelectric phase. A soft-lattice-mode model does not quantitatively describe the temperature dependence of the frequency shifts. A model which explicitly takes into account the protonic disorder accounts quantitatively for the arsenic data. No quantitative treatment that is in good agreement with the rubidium data has been found.