Potential energy maps and H+ conduction mechanism in HTaWO6•xH2O pyrochlore

Least-energy paths of proton in the structures of HTaWO6 and HTaWO6·H2O have been calculated by the Born model of crystal energy. The atomic charge on O and the repulsive parameters are fitted to reproduce the equilibrium crystal structure of the pyrochlore KNbWO6, obtaining zo=−0.70 e. Maps of H+ potential energy in the anhydrous compound show that proton hopping occurs between adjacent H sites along edges of (Ta, W)O6 coordination octahedra, with a theoretical activation energy of 0.60 eV in good agreement with the experimental value (0.66 eV). In the hydrated compound the proton transfer occurs by a similar mechanism, excluding an active role of water molecules. The small experimental activation energy observed in the latter case is probably due to surface conduction induced by humidity at grain boundaries