Investigations of the Phase Transition and Proton Dynamics in Rubidium Methane Phosphonate Studied by Solid-State NMR

In search of new solid acid proton conductors, we prepared the solid acid rubidium methane phosphonate (RMP). These crystals have a monoclinic structure (P2/c; a = 9.3452, b = 9.3142, and c = 7.5021 Å; β = 101.12). The salt incorporates a hydrated lamellar structure. The 1H MAS NMR reveals two different types of acidic protons as well as the water protons in the lamella. The 1H VT MAS NMR of RMP·2H2O single crystal shows a structural phase transition around 320 K, and the high-temperature phase exhibits significant proton dynamics. The proton proximities are established by solid state 1H DQF NMR. The dehydration of RMP crystal leads to structural collapse, and the resultant RMP powder is extremely hygroscopic. The proton environment and dynamics are examined using 1H DQF NMR, which reveals that the dehydrated RMP powder has rigid lattice, in contrast with the hydrated form. Further the 1H VT MAS NMR shows that dehydrated RMP powder has no phase transition, and no significant proton dynamics are observed in the temperature range of 250−350 K. The new hydrated crystal, RMP·2H2O, shows high proton mobility at relatively low temperature (∼330 K) and a proton transport mechanism that uniquely relies on crystalline water.