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 angstrom; beta = 101.12). The salt incorporates a hydrated lamellar structure. The H-1 MAS NMR reveals two different types of acidic protons as well as the water protons in the lamella. The H-1 VT MAS NMR of RMP center dot 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 H-1 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 H-1 DQF NNIR, which reveals that the dehydrated RMP powder has rigid lattice, in contrast with the hydrated form. Further the H-1 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 center dot 2H(2)O, shows high proton mobility at relatively low temperature (similar to 330 K) and a proton transport mechanism that uniquely relies on crystalline water.