On the meaning of effective formation energies, entropies and volumes for atomic defects in ordered compounds

The statistical mechanics of atomic defects in ordered compounds is discussed within the framework of a generalized grandcanonical ensemble. To do this, grandcanonical defect formation parameters are introduced which characterize the properties of the respective single defects and which may be calculated by the ab initio electron theory. It is demonstrated that the commonly used effective formation parameters, defined via the temperature and pressure dependence of the defect concentrations, depend on the grandcanonical defect formation parameters of all the defects involved. They are in general complicated functions of the composition and do not have a simple physical meaning as in monoatomic crystals. This must he taken into account to avoid serious misinterpretations of experimental results. (C) 1997 Acta Metallurgica Inc.