A rather strong and unusual temperature and frequency dependence of the proton spin-lattice relaxation time $T_1$ has been observed in a ${\mathrm{Rb}}_3{\mathrm{H}(\mathrm{S}\mathrm{O}}_4{)}_2$ single crystal in the temperature interval between 300 and 10 K. Below 10 K the proton $T_1$ becomes temperature independent but still exhibits a rather strong Larmor frequency dependence. At higher temperatures the spin-lattice relaxation is determined by a modulation of the Rb-proton and/or proton-proton dipolar coupling due to thermally activated jumping of the proton between the two equilibrium sites in the H bond whereas at low temperatures phonon-assisted proton incoherent tunneling takes over. The results thus show that the double-minimum potential of the H bond in ${\mathrm{Rb}}_3{\mathrm{H}(\mathrm{S}\mathrm{O}}_4{)}_2$ is locally asymmetric 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.

• Received 26 May 1999

DOI:https://doi.org/10.1103/PhysRevB.61.197