Protonic inter-H-bond motion and ionic conductivity in hydrogen-bonded proton glasses

Abstract

Low-frequency dynamics of the hydrogen-bonded proton glass ${\mathrm{Rb}}_{1\ensuremath{-}x}{(\mathrm{N}\mathrm{H}}_{4}{)}_{x}{\mathrm{H}}_{2}{\mathrm{AsO}}_{4}$ and its deuterated form has been investigated by measuring the decay of the NMR transverse nuclear-spin magnetization of ${}^{87}\mathrm{Rb}$ nuclei in a spatially inhomogeneous electric-field gradient, ${}^{1}\mathrm{H}$ line shape and ${}^{2}\mathrm{H}$ two-dimensional exchange NMR techniques. A large difference in the time scales of the decay of Hahn echo and Carr-Purcell echo train was observed between room temperature and 10 K. The Hahn echo decay curves showed an exponential decay with the exponent proportional to the cube of time that is characteristic for random stochastic atomic motions. The proton NMR spectrum demonstrates the presence of mobile protons which produce a motionally narrowed absorption line above 220 K. Two-dimensional exchange NMR experiment on the deuterated glass demonstrated the existence of slow thermally activated deuteron hopping between different hydrogen bonds, the process that represents the ionic conductivity. The same effects were observed also in several pure compounds of the ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ family. The observed low-frequency dynamics is not a peculiarity of the proton glass phase but represents the protonic inter-H-bond motion and ionic conductivity that is a common feature of the ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ family.