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,CsH5(PO4)2, by means of nuclear magnetic resonance (NMR) spectroscopy, in order to address thequestion of why there is no superprotonic phase transition in this compound, in contrast toother structurally similar hydrogen-bonded ionic salts, where a superprotonic transition isfrequently found to be present. The analysis of the NMR spectrum and the spin–latticerelaxation rate revealed that the temperature-dependent hydrogen dynamics ofCsH5(PO4)2 involves motional processes (the intra-H-bond jumps and the inter-H-bond jumps atelevated temperatures, as a mechanism of the ionic conductivity) identical to those for theother H-bonded superprotonic salts. The considerably stronger H-bond network inCsH5(PO4)2 prompts the search for a higher superprotonic transition temperature. However, dueto the relatively weak bonding between the planes in the [100] direction of theCsH5(PO4)2 structure by means of the ionic bonding via the cesium atoms and the small number ofH bonds in that direction (where out of five H bonds in the unit cell, four aredirected within the planes and only one is between the planes), the bonds between the planes becomethermally broken and the crystal melts before the H-bond network rearranges viawater release into an open structure typical of the superprotonic phase. Were thecoupling between the planes in the CsH5(PO4)2 somewhat stronger, the superprotonic transition would occur in the same manner as it doesin other structurally related hydrogen-bonded ionic salts.