Rb87 and As75 Quadrupolar Coupling in Ferroelectric RbH2PO4 and CsH2AsO4

Abstract

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.