Proton (deuteron) conductivity in Cs1.5Li1.5H(SO4)(2) and Cs1.5Li1.5D(SO4)(2) single crystals

Calorimetric (DSC), NMR and electrical studies of Cs1.5Li1.5X(SO4)(2) (X = H, D) single crystals have been performed in the temperature range from 300 to 533 K. No phase transitions are observed upon heating and the conductiv ity follows an Arrhenius temperature dependence to the point of decompositi on at similar to 470 K. Despite the high protonic conductivity of Cs1.5Li1. 5X(SO4)(2) (sigma similar to 10(-3) S cm(-1)) at high temperature, these co mpounds cannot be classified as superprotonic because of the large value of the activation energy (E-a similar to 1 eV), The proton NMR studies confir med that two crystallographic proton sites exist in Cs1.5Li1.5H(SO4)(2), pr esumably corresponding to two minima in the single, crystallographically di stinct (and asymmetric) hydrogen bond. A measurable isotope effect in the c onductivity data confirmed that protons/deuterons, as opposed to lithium io ns, are the mobile species. The high activation energy for proton transport in CS1.5Li1.5X(SO4)(2) Most probably results from the asymmetry of the hyd rogen bonds, and from electrostatic repulsion between the protons and the L i+ ions which likely hinders reorientation of XSO4- groups. (C) 2001 Publis hed by Elsevier Science B.V.