Orientational defects on a hydrogen-bonded chain

In hydrogen-bonded crystals such as ice, it has been proposed that charge transport occurs through a process involving two types of defects (the so-called orientational and ionic defects) in the proton sublattice. We have investigated the formation and dynamics of the orientational defects through two approaches: a quantum-chemical study of the barrier to rotation in finite chains of H₂O molecules, and a study, based on an extended tight-binding model, of the formation and motion of these defects. We find from the first part of this study that the barrier to rotation may be nearly an order of magnitude lower than the barrier to proton hopping along the chain. The subsequent construction of the modified tight-binding model allows us to simulate motion in a uniform electric field of a pair of (charged) orientational defects. Calculated mobilities for the more mobile positively charged defect are in the range 0.39-0.46 cm²/V s for a dielectric screening constant of 50, in agreement with available experimental values.