DYNAMICS OF PROTONIC CONDUCTORS RB3H(SO4)(2) AND RB3D(SO4)(2) STUDIEDBY NMR

The dynamics of Rb3H(SO4)(2) and Rb3D(SO4)(2) crystals has been studie d between 4 K and the superionic phase (T-c approximate to 448 K). Rb- 87 and deuteron NMR spectra exhibit motional narrowing on heating wher eas the spin-lattice relaxation (T-1) rates behave in an unusual way-t hey increase on cooling in the whole investigated temperature range bu t show no frequency dependence. Such a behavior can be explained by th e phonon induced relaxation via Raman processes. The results demonstra te that the dynamics of the above systems is predominantly determined by the thermally induced SO4 vibrations, torsional oscillations, and r eorientations resulting from the high elasticity of the crystal struct ure. Lattice vibrations dominate crystal dynamics in a large temperatu re range from the superionic phase down to as low as 20 K. In addition the deuteron soft-mode slowing-down dynamics has been observed close to the antiferroelectric transition in the deuterated compound. The de uteron two-dimensional exchange NMR experiment has detected the deuter on interbond motion as a precursor of the superionic conductivity alre ady at room temperature which is as much as 170 K below the superionic phase transition.