title: The hydrogen dynamics of CsH5(PO4)2 studied by means of nuclear magnetic resonance.
abstract: We have investigated the hydrogen dynamics of cesium pentahydrogen diphosphate, CsH(5)(PO(4))(2), by means of nuclear magnetic resonance (NMR) spectroscopy, in order to address the question of why there is no superprotonic phase transition in this compound, in contrast to other structurally similar hydrogen-bonded ionic salts, where a superprotonic transition is frequently found to be present. The analysis of the NMR spectrum and the spin-lattice relaxation rate revealed that the temperature-dependent hydrogen dynamics of CsH(5)(PO(4))(2) involves motional processes (the intra-H-bond jumps and the inter-H-bond jumps at elevated temperatures, as a mechanism of the ionic conductivity) identical to those for the other H-bonded superprotonic salts. The considerably stronger H-bond network in CsH(5)(PO(4))(2) prompts the search for a higher superprotonic transition temperature. However, due to the relatively weak bonding between the {[H(2)PO(4)]}∞ planes in the [100] direction of the CsH(5)(PO(4))(2) structure by means of the ionic bonding via the cesium atoms and the small number of H bonds in that direction (where out of five H bonds in the unit cell, four are directed within the {[H(2)PO(4)]}∞ planes and only one is between the planes), the bonds between the planes become thermally broken and the crystal melts before the H-bond network rearranges via water release into an open structure typical of the superprotonic phase. Were the coupling between the {[H(2)PO(4)]}∞ planes in the CsH(5)(PO(4))(2) somewhat stronger, the superprotonic transition would occur in the same manner as it does in other structurally related hydrogen-bonded ionic salts.
authors.affiliation: J Stefan Institute, University of Ljubljana, Jamova 39, SI-1000 Ljubljana, Slovenia.
authors.forename: A
authors.initials: A
authors.lastname: Gradišek
authors.forename: B
authors.initials: B
authors.lastname: Dimnik
authors.forename: S
authors.initials: S
authors.lastname: Vrtnik
authors.forename: M
authors.initials: M
authors.lastname: Wencka
authors.forename: M
authors.initials: M
authors.lastname: Zdanowska Fraczek
authors.forename: G V
authors.initials: GV
authors.lastname: Lavrova
authors.forename: J
authors.initials: J
authors.lastname: Dolinšek