87 Rb NMR spectra of hydrogen-bonded Rb 3 D ( S O 4 ) 2 and Rb 3 H ( S O 4 ) 2 crystals

The paraelectric and antiferroelectric phases of ${\mathrm{Rb}}_{3}{\mathrm{D}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ and ${\mathrm{Rb}}_{3}{\mathrm{H}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ were investigated using rubidium NMR. The angle dependence of the second order quadrupole shifted central ${}^{87}\mathrm{Rb}$ 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 ${\mathrm{Rb}}_{3}{\mathrm{D}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ ${(T}_{N}=82\mathrm{K})$ is associated with a quadrupling of the unit cell. Below ${T}_{N}$ 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.