2 H NMR investigation of the transition to the proton glass state in the Cs 5 H 3 (SO 4 ) 4 · 0.5H 2 O crystal

A crystal of the Cs 5 H 3 (SO 4 ) 4 · x H 2 O ( x ≈ 0.5) (PCHS) compound, which belongs to the family of proton conductors with a complex system of hydrogen bonds, is investigated by 2 H NMR spectroscopy. The temperature and orientation dependences of the 2 H NMR spectra are measured and analyzed. It is established that, upon transition to the glassy phase at the temperature T g = 260 K, the parameters characterizing the proton exchange between positions in hydrogen bonds remain unchanged to within the limits of experimental error. The protons in the two-dimensional network of hydrogen bonds in the (001) plane are dynamically disordered over possible positions down to temperatures considerably lower than the glass transition point T g . However, water molecules are fixed at particular structural positions in the phase transition range. In PCHS crystals with a nonstoichiometric water content, this circumstance can be responsible for the frustration that leads to the formation of the glassy state.