Phase transitions and proton dynamics in [(NH4)1−xRbx]3H(SO4)2 (0 < x < 1) were studied by means of differential scanning calorimetry, thermogravimetric and differential thermal analyses (TG-DTA) and 1H solid-state nuclear magnetic resonance. Thermal analyses demonstrate that a phase transition from a room-temperature phase (phase II) to a high-temperature phase (phase I) takes place. The transition temperature increased with x. On the other hand, the melting/decomposition temperature was almost independent of x. Thus, the

temperature range of phase I become narrower with x. The 1H magic-angle-spinning nuclear magnetic resonance spectra at room temperature demonstrate that the acidic protons form the stronger H bond with increase in x. This trend might correlate with the transition from phases II to I. 1H static nuclear magnetic resonance spectra indicate that the proton motions in [(NH4)1−xRbx]3H(SO4)2 (0 < x < 1) were similar to those in (NH4)3H(SO4)2 and Rb3H(SO4)2, although the temperature ranges were different. In phase I, both NH4+ ions and the acidic protons diffuse translationally, which supports that phase I was super-protonic.

Mixed-Cation Effect in a Superprotonic Phase of [(NH4)1−xRbx]3H(SO4)2 Studied by 1H Solid-State NMR. H.Omi, K.Suzuki, S.Hayashi: Solid State Ionics, 2008, 179[17-18], 599-604