87Rb-NMR spectra of the hydrogen-bonded crystal Rb3D(SO4)2

Abstract

The paraelectric and antiferroelectric phases of Rb3D(SO4)2 and Rb3H(SO4)2 were investigated using rubidium NMR. The angle dependence of the second order quadrupole shifted central 87Rb transitions was recorded at 300 K. It was found to be in full agreement with the symmetry elements of a monoclinic A2/a structure. The temperature dependence of the magnitude and asymmetry of the electrical field gradient (EFG) tensors at the Rb sites was determined using powder samples and revealed no differences between protonated and deuterated specimens. The antiferroelectric transition of Rb3D(SO4)2 (TN=82 K) is associated with a quadrupling of the unit cell. Below TN the magnitudes of the EFG at the Rb sites remain constant and smoothly tilt away from their high temperature orientations. The temperature dependence of the NMR line splittings associated with this tilt follows a power law implying an order parameter critical exponent of 0.21.