Proton spin-lattice relaxation and local symmetry of the H bond in Rb3H(SO4)(2)

A rather strong and unusual temperature and frequency dependence of the pro ton spin-lattice relaxation time T-1 has been observed in a Rb3H(SO4)(2) Si ngle crystal in the temperature interval between 300 and 10 K. Below 10 K t he proton TI becomes temperature independent but still exhibits a rather st rong Larmor frequency dependence. At higher temperatures the spin-lattice r elaxatian is determined by a modulation of the Rb-proton and/or proton-prot on dipolar coupling due to thermally activated jumping of the proton betwee n the two equilibrium sites in the H bond whereas at low temperatures phono n-assisted proton incoherent tunneling takes over. The results thus show th at the double-minimum potential of the H bond in Rb3H(SO4)(2) is locally as ymmetric in the temperature range investigated. There is no evidence for a change from a double to a single minimum type H-bond potential down to 4 K.