Long range migration of ionic charge carriers, the most mobile species, through the lattice under application of an electric field for example migration of Na+ ions in soda-silicate glasses

Dependent on the presence of vacant sites in neighbourhood of mobile defects/ions.

Can occur through grain boundaries such as in polycrystalline ceramics or through the lattice as in fast ion conductors.

When external field is absent, the thermal energy, kT, is required for counter migration of ions and vacancies overcoming the migration energy Ea, as shown in Figure 3.5, which is nothing but process of self diffusion.

In the presence of electric field, the potential energy is tilted to one side leading to higher driving force for migration towards one side than to another side (see figure 3.4). The field is small enough not to pull the ions out, rather it is enough to tilt the energy well such that ions move in one direction preferentially.

Ionic conductivity is promoted by

• Small ionic size
  
  
• Small charge i.e. less Coulomb interaction between ions
  
  
• Favourable lattice geometry
  
  
• Cations are usually smaller than anions and hence, they diffuse faster. For example, in case of NaCl, smaller size of Na⁺ ion (102 pm) as compared to Cl^- ions (181 pm) makes them diffuse faster.