Proton diffusion in the room-temperature phase of [(NH4)1−xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation in the rotating frame

Proton diffusion in the room-temperature phase (phase II) of [(NH4)1−xRbx]3H(SO4)2 (0≤x≤1) has been studied by means of 1H spin-lattice relaxation times in the rotating frame, T1ρ. The 1H T1ρ values were measured at 200.13 MHz in the range of 380-490 K. The ammonium protons and the acidic protons have independent T1ρ values in the higher temperature range of phase II, suggesting that the spin diffusion between the two species is ineffective. The translational diffusion of the acidic protons is the most dominant mechanism to relax both the ammonium protons and the acidic protons in phase II. The 1H T1ρ values in phase II are analyzed theoretically and the motional parameters are obtained. The results of NMR well explain the macroscopic proton conductivity.