  A thermodynamic model based on the law of mass action is used to calculate
concentrations of elementary point defects and to determine site preferences of
solute atoms in ordered alloys. Combinations of lattice vacancies, antisite
atoms and host interstitials that form equilibrium defects are enumerated for
the CsCl (B2) and Ni2Al3 structures. For CsCl, in addition to the two
substitutional sites, a distorted tetrahedral interstitial site is considered.
For Ni2Al3, the Ni site, two distinct Al sites and a vacant, insterstitial-type
Ni-site are considered. An equation of constraint among concentrations of
elementary defects is derived that is valid for any crystal structure. The
concentration of a selected defect can be solved using the equation of
constraint in conjunction with mass-action equations for defect combinations.
The method leads directly to defect concentrations without the need to evaluate
composition-dependent chemical potentials, resulting in a more transparent
formalism.
  The model is used to explore the phenomenology of site-preferences of dilute
ternary solute atoms. Findings are in agreement with previous treatments
restricted to substitutional sites. General rules for how site-preferences
depend on temperature and on composition in non-stoichiometric compounds are
obtained through algebraic analysis and numerical simulations: (1) Solute S
tends to occupy substitutional sites of the element in which there is a
deficiency. (2) If the difference of energies of S on sites A and B is very
positive or negative, then S will occupy site B or A exclusively, independent
of composition. If the difference of site energies is intermediate, the solute
will switch from one site to the other as the composition changes. (3) Solutes
have a tendency to occupy interstitial or empty-lattice sites with a maximum
site-fraction near the stoichiometric composition.
