Deuteron NMR and the local symmetry of the H bond in Rb3D(SO4)(2)

The temperature and angular dependencies of the deuteron NMR spectra of Rb3 D(SO4)(2) have been measured and the electric-field gradient (EFG) tensors at the deuteron sites have been determined both above and below the antifer roelectric transition temperature T-c. In contrast to the Rb-87 1/2 --> -1/ 2 NMR spectra no pronounced deuteron line splitting has been found at T-c. This seems to show that there is a center of symmetry at the midpoint of th e O-D ... O bond. The above results are compatible with the centrosymmetric space group A2/a and a doubling of the unit-cell axes along the b and c di rections below T-c. The appearance of a small, but definite, anomaly in the deuteron spin-lattice relaxation T-1 at T-c on the other hand suggests tha t the deuteron motion between the two off-center sites in the O-D ... O bon d is connected with electric-field gradient tensor fluctuations. The H-bond potential must be thus slightly asymmetric so that the true space group is A2 both above and below T-c. The deviations from A2/a must be, however, ra ther small. The H-bond asymmetry fluctuates in space and time due to deuter on motion so that the time- and space-average crystal symmetry seen by x ra ys is indeed A2/a. [S0163-1829(99)04317-9].