Revealing Local Dynamics of the Protonic Conductor CsH(PO<inf>3</inf>H) by Solid-State NMR Spectroscopy and First-Principles Calculations

23 May 2018

A joint study incorporating multinuclear solid-state NMR spectroscopy and first-principles calculations has been performed to investigate the local structure and dynamics of the protonic conductor CsH(PO3H) in the paraelectric phase. The existence of the superprotonic phase (>137 oC) is clearly confirmed by NMR, in good agreement with the literature. The variable-temperature 1H, 2H and 31P NMR data further reveal a distribution of motional correlation times, isotropic rotation of the phosphite ion being observed below the superprotonic phase transition for a small but gradually increasing subset of anions. This isotropic rotation is associated with fast local protonic motion, the distribution of correlation times being tentatively assigned to internal defects or surface absorbed H2O. The phosphite ion dynamics of the majority slower subset of phosphite ions is quantified through analysis of variable-temperature 17O spectra recorded from 34 to 150 ºC, by considering a model for the pseudo C3 rotation of the phosphite ion around the P-H bond axis below the phase transformation. An extracted activation energy of 0.24 ± 0.08 eV (23 ± 8 kJ mol-1) for this model was obtained, much lower than that reported from proton conductivity measurements, implying that no strong correlation exists between long range protonic motion and C3 rotations of the phosphite. We conclude that proton conduction in CsH(PO3H) in the paraelectric phase is governed by both the activation energy for exchange between donor and acceptor oxygen sites, rotation of the phosphite units, and the lack of isotropic rotation of the phosphite ion. Surprisingly, coalescence of 17O NMR resonances, as would be expected for rapid isotropic reorientations of all phosphite groups, is not observed above the transition. Potential reasons for this are discussed.