Rb87Quadrupole Coupling and the Ferroelectric Transition in RbH2AsO4

Abstract

The quadrupole-induced shifts of the ${\mathrm{Rb}}^{87}$ \textonehalf{} \ensuremath{\rightarrow} -\textonehalf{} NMR transition have been studied as a function of temperature and crystal orientation, and the electric-field-gradient (EFG) tensors at the Rb sites in Rb${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$ have been determined. In the ferroelectric phase there are two physically nonequivalent ${\mathrm{Rb}}^{87}$ EFG tensors ($A$ and $B$) which are related by the symmetry operations of the orthorhombic space group. In addition, each line is split into two components ($A$, ${A}^{\ensuremath{'}}$ and $B$, ${B}^{\ensuremath{'}}$) because of domain effects. The ${\mathrm{Rb}}^{87}$ quadrupole coupling constant equals 5.76 MHz at 108\ifmmode^\circ\else\textdegree\fi{}K, and the asymmetry parameter is $\ensuremath{\eta}==0.85$. The paraelectric EFG tensor is nearly, though not exactly, equal to the average of the two ferreolectric ones ($A$ and ${B}^{\ensuremath{'}}$, or ${A}^{\ensuremath{'}}$ and $B$) from oppositely polarized domains. The anomalous decrease in the quadrupole coupling constant of ${\mathrm{Rb}}^{87}$ with decreasing temperature in the paraelectric phase throws some light on the atomic motion as well as proton-lattice coupling and suggests that the Rb nuclei take part in collective atomic fluctuations which might represent an overdamped quasispin-wave ferroelectric mode.