Investigations of Proton Conduction in the Monoclinic Phase of RbH2PO4 Using Multinuclear Solid-State NMR

To understand the nature of the high-temperature phase transition and the origin of proton conductivity in rubidium dihydrogen phosphate RbH2PO4, (RDP), we utilized variable-ternperature H-1, P-31, and Rb-87 MAS NMR at 11.7 and 21.1 T. The variable-temperature H-1 MAS NMR spectra do not show water peaks in the temperature range of 300-400 K, which argues against a partial polymerization and/or decomposition. The H-1 DQ MAS NMR spectra acquired at high field (21.1 T) reveal three different proton sites, and dipolar recoupling experiments give estimates of distances. This evidence supports the formation of a superstructure (doubling of the unit cell along the a-axis) in the high-temperature monoclinic phase. The variable-temperature proton spin-lattice relaxation (T-1) reveals significant proton dynamics above 340 K, coinciding with the temperature at which an increase of bulk proton conductivity is reported in monoclinic RDP, but well below the macroscopic phase transition. These precursor dynamics foreshadow the onset of superprotonic conductivity. From variable-temperature P-31 and Rb-87 MAS NMR spectra, it is deduced that significant proton dynamics above 340 K are due to the proton transfer between hydrogen bonds accompanied by reorientation of PO4 tetrahedra.