2.8 Intrinsic and Extrinsic Defects

       2.8.1 Intrinsic behavior

• Defect which can be determined from the intrinsic defect equation and is temperature dependent, increasing with increasing temperature.

       2.8.2 Extrinsic behavior

• Extrinsic defects are defects caused by impurities consisting of aliovalent cations.

• Defect concentration depends upon impurity concentration which is constant and independent of temperature. Only at very high temperatures, intrinsic behavior again dominates, and the cross-over temperature depends upon the defect formation energy.

       2.8.3 Example

Defect formation energies for some ceramic materials are

Here, one can see the relation with the melting point that melting point of MgO is ~2825°C while it is ~801°C for NaCl. So, at any given temperature NaCl will have much larger defect concentration than MgO. However, at the same   homologous temperature, defect concentrations can be quite similar.

Interestingly, while the highest achievable purity level in MgO is 1 ppm, in NaCl, it is 50 ppm. Typically, these impurities consist of aliovalent cations which give rise to defects, called extrinsic defects. Thus the concentration of extrinsic defects is much greater than intrinsic defect concentration in MgO.  As a result, defects in NaCl are likely to be intrinsic but MgO is most likely to contain extrinsic defects.