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 ( P 2/ c; a = 9.3452, b = 9.3142, and c = 7.5021 Å; β = 101.12). The salt incorporates a hydrated lamellar structure. The 1 H MAS NMR reveals two different types of acidic protons as well as the water protons in the lamella. The 1 H VT MAS NMR of RMP·2H 2 O 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 1 H 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 1 H DQF NMR, which reveals that the dehydrated RMP powder has rigid lattice, in contrast with the hydrated form. Further the 1 H 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·2H 2 O, shows high proton mobility at relatively low temperature (∼330 K) and a proton transport mechanism that uniquely relies on crystalline water.