Proton spin-lattice relaxation and local symmetry of the H bond inRb3H(SO4)2

Abstract

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.