H1NMR study of proton dynamics inCs5H3(SO4)4∙xH2O

Abstract

Complicated phase relations in ${\mathrm{Cs}}_{5}{\mathrm{H}}_{3}{(\mathrm{S}{\mathrm{O}}_{4})}_{4}∙x{\mathrm{H}}_{2}\mathrm{O}$ are revealed by thermal analyses. A superprotonic phase transition takes place at $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for both the hydrated and the anhydrous forms. The $^{1}\mathrm{H}$ magic-angle-spinning (MAS) NMR spectra were traced at room temperature and at Larmor frequency of $400.13\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. The $^{1}\mathrm{H}$ chemical shifts of the acidic protons are 10.9 and $11.2\phantom{\rule{0.3em}{0ex}}\mathrm{ppm}$ for the hydrated and the anhydrous forms, respectively. The hydrated sample shows a signal at $9.7\phantom{\rule{0.3em}{0ex}}\mathrm{ppm}$ additionally, which is ascribed to ${\mathrm{H}}_{3}{\mathrm{O}}^{+}$. Proton dynamics has been studied by $^{1}\mathrm{H}$ static NMR spectra and spin-lattice relaxation times, ${T}_{1}$. In both a room-temperature phase (phase RT) and a high-temperature phase (phase HT), translational diffusion of protons takes place. The $^{1}\mathrm{H}$ mean residence times in phase RT are obtained from the second moment analysis; ${E}_{\mathrm{a}}=49\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}$ and ${\ensuremath{\tau}}_{0}=1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ for the anhydrous form and ${E}_{\mathrm{a}}=31\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}$ and ${\ensuremath{\tau}}_{0}=2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ for the hydrated form. From the analysis of $^{1}\mathrm{H}$ ${T}_{1}$ results we have obtained parameters of proton diffusion for phase HT; ${E}_{\mathrm{a}}=34\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}$ and ${\ensuremath{\tau}}_{0}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ for the anhydrous form and ${E}_{\mathrm{a}}=36\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}$ and ${\ensuremath{\tau}}_{0}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ for the hydrated form. In both phases, protons diffuse faster in the hydrated form than in the anhydrous form. The proton conductivities estimated from the NMR results for the anhydrous form agree with the macroscopic values in literature.