Equilibrium vacancy concentrations in non-stoichiometric B2 compounds by Monte Carlo simulations

Results are reported from Monte Carlo simulations of non-stoichiometric compounds with the B2 structure and which contain a high concentration of vacancies. The model chosen is a rigid lattice with nearest-neighbour atom interactions only. The vacancy concentrations evaluated correspond with two-phase equilibrium and not those obtained by minimising the single phase free energy. The partial thermodynamic properties of the components are evaluated along the phase equilibrium boundary together with the concentrations of defects on the two sub-lattices. The triple-defect assumption, frequently used in analytical treatments for vacancies in these compounds, is shown to be of very limited value.