Dynamics of the superprotonic conductor K9H7(SO4)8•H2O studied by means of nuclear magnetic resonance

The dynamics of a recently discovered superprotonic conductor K9H7 (SO4)(8) H2O has been studied between 40 and 425 K by techniques based on the NMR spectrum shape, spin-lattice and spin-spin relaxation. At low temperatures (such as 40 K), proton intra-H-bond hopping is already intensive. At higher temperatures, water 180degrees reorientations become observable in the NMR experiments, whereas above 250 K, proton interbond jumps - a precursor of the superprotonic conductivity above the transition temperature T-sp = 398 K - become frequent. Above T-sp, a large increase in the proton spin-spin relaxation time T-2 indicates that proton long-range diffusion becomes significant. Proton interbond jumps are assisted by reorientations of the SO4 tetrahedra, which also cause breaking of the water bonds, so water molecules become free and consequently diffuse out of the crystal. The loss of water allows rearrangement of the lattice, so the number of structurally equivalent proton sites in the superprotonic phase is increased, resulting in a very open structure for the hydrogen interbond transfer.