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.