A study of the phase transitions and proton dynamics of the superprotonic conductor Cs5H3(SO4)4·0.5H2O single crystal with 1H and 133Cs nuclear magnetic resonance

The phase transitions and proton dynamics of Cs5H3(SO4)4·0.5H2O single crystals were studied by measuring the NMR line shape, the spin-lattice relaxation time, T1, and the spin–spin relaxation time, T2, of the 1H and 133Cs nuclei. The “acid” protons and the “water” protons in Cs5H3(SO4)4·0.5H2O were distinguished. The loss of water protons was observed above TC1, whereas the content of water protons was found to recover above TC2. Therefore, the water protons play a special role in the stability of the superprotonic phase at high temperatures. The mechanism of fast proton conduction was found to consist of hydrogen-bond proton transfer involving the breakage of the weak part of the hydrogen bond and the formation of a new hydrogen bond. Thus, these structural phase transitions probably involve significant reorientation of the SO4 tetrahedra and dynamical disorder of the hydrogen bonds between them.