Point defects, electrons, and holes as chemical species

7 Point defects, electrons, and holes as chemical species 

So far we have derived an expression for the free energy of the solid phase under consideration after Libowitz, and discussed the composition or temperature dependence of the activity derived from this expression of free energy. Let us now consider non-stoichiometry from another point of view. 

3 STATISTICAL THERMODYNAMICS OF POINT DEFECTS hydrogen sites, respectively. Hence we have 

Consider a metal hydride MH in which there are X lattice points for metals and sN for hydrogens, and hydrogen atoms occupy Nh in sN lattice points (sN = + Ny, where A/y is the number of vacant hydrogen sites). 

Next let us discuss the electronic defects associated with point defects in semiconductive or insulating compounds, which lead to non-stoichiometry. Consider a NiO crystal, which has a NaCl-type structure, as NiO can be regarded as an ionic crystal, the valence states of Ni and O are Ni and O , respectively. We assume that the non-stoichiometry originates only from metal vacancies. Generation of metal defects in NiO may be expressed by a chemical reaction similar to eqn (1.119), i.e. 

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