RB87 QUADRUPOLE COUPLING AND FERROELECTIC TRANSITION IN RBH2ASO4

The quadrupole-induced shifts of the Rb87 1/2 → -1/2 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 RbH2AsO4 have been determined. In the ferroelectric phase there are two physically nonequivalent Rb87 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′ and B, B′) because of domain effects. The Rb87 quadrupole coupling constant equals 5.76 MHz at 108°K, and the asymmetry parameter is η==0.85. The paraelectric EFG tensor is nearly, though not exactly, equal to the average of the two ferreolectric ones (A and B′, or A′ and B) from oppositely polarized domains. The anomalous decrease in the quadrupole coupling constant of Rb87 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. © 1969 The American Physical Society.