4 - Defects, non-stoichiometry and phase transitions

Summary

Introduction

Protonic conduction can be considered as a particular case of ionic conduction; however, there are some similarities with electronic conduction because of the proton size. In both cases, defects can play an important role.

Ionic conductivity in a compact structure is generally very low and requires a high activation energy. The diffusion coefficient of oxygen in amorphous silica at 1000°C, for instance, is 10^–14cm₂ s^–1 and that in MgO is 10^–20 cm² s^–1, with activation energies between 1 and 4 eV. In single crystal and polycrystalline alumina at 1700 °C, the diffusion coefficient decreases to 10^–15 and 10^–12cm² s^–1, respectively, and the activation energy increases to 6–8 eV. The diffusion coefficients of smaller cations such as Al³⁺ are of the order of 10^–9–10^–10 cm² s^–1 with activation energy of about 5 eV.

The diffusion coefficient of Al³⁺ ions in interstitial sites can be obtained from electrical conductivity measurements. Its activation energy is considerably lower (2.5 eV) than that of lattice diffusion. This applies not only to Al³⁺ in alumina but generally to all atoms occupying defect sites. All compounds have naturally a certain number of intrinsic defects as well as those associated with impurities (extrinsic defects). Intrinsic defects can be thermally activated and are particularly important for non-stoichiometric compounds. In the latter, diffusion coefficients are generally higher and the activation energy lower than that in stoichiometric compounds.