The physics of defect chemistry and the chemistry of defect physics

Defect chemistry is the classical approach to evaluate point-defect concentrations in solids depending on the chemical activity of the (n - 1) of n constituents by evaluating the mass action laws of a number of defect reactions conserving species, lattice sites, and charge. In an alternative approach, formation energies of individual defects can be calculated to determine the dependence on the Fermi level and on the chemical potentials of the reservoirs. This contribution provides the quantitative relationship between the two approaches, offering the opportunity to compare calculated defect formation energies with experimentally determined quantities. As an example, the application of the two approaches to the comparison of electronic and ionic compensation of doping and the influence of the band edge energies on it is given. This example demonstrates that the Gibbs energy of reduction and oxidation are essentially aligning the energy axis of ionic defects relative to that of electronic defects. In conjunction with the dependence of the valence band maximum and conduction band minimum energies on material composition, this offers the opportunity to tune the preference for electronic, ionic or mixed compensation of doping by two independent quantities.