Dynamics Of Protonic Conductors Rb3h(So4)(2) And Rb3d(So4)(2) Studied By Nmr

The dynamics of Rb3H(SO4)(2) and Rb3D(SO4)(2) crystals has been studied between 4 K and the superionic phase (T-c approximate to 448 K). Rb-87 and deuteron NMR spectra exhibit motional narrowing on heating whereas the spin-lattice relaxation (T-1) rates behave in an unusual way-they increase on cooling in the whole investigated temperature range but show no frequency dependence. Such a behavior can be explained by the phonon induced relaxation via Raman processes. The results demonstrate that the dynamics of the above systems is predominantly determined by the thermally induced SO4 vibrations, torsional oscillations, and reorientations resulting from the high elasticity of the crystal structure. Lattice vibrations dominate crystal dynamics in a large temperature 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 deuteron two-dimensional exchange NMR experiment has detected the deuteron interbond motion as a precursor of the superionic conductivity already at room temperature which is as much as 170 K below the superionic phase transition.