Motion of ionic and orientational defects in a hydrogen-bonded chain

Abstract

We have constructed simple models which allow us to investigate the formation and motion of both ionic and orientational defects in a chain of hydrogen-bonded sites. The models incorporate a classical lattice hamiltonian and a modified tight-binding electronic hamiltonian in which site energies are dependent on lattice displacements. Numerical simulations of the dynamics have been carried out on 100-site chains in both the absence and presence of an electric field. The models predict that the barrier to rotations is nearly an order of magnitude smaller than the hopping barrier in a chain of H 2O molecules. The hopping barrier diminishes under pressure, disappearing when the OO distance is reduced by ∼13% from its physical equilibrium value of 2.75 A. Stable defect pairs of either type can be created; equilibrium defect width is only a few sites. In the case of orientational defects, physically reasonable fields can lead to defect motion; for ionic defects, however, coherent defect motion is seen only at extremely large field values.