Abstract
Thermogravimetric measurements of the compositional phase diagram of 
were made as a function of temperature from 1000° to 1500°C and as a function of oxygen partial pressure over most of the stable phase field. The values of 
in 
are presented as a function of temperature and oxygen pressure both numerically and graphically. The pressure dependence of the vacancy concentration at constant temperature was found to obey a relation of the form 
. The enthalpy of solution of oxygen in this composition range is about −0.3 eV. At larger departures from stoichiometry, a sharp transition to a different defect structure is observed in the 
plots, and the enthalpy of oxygen dissolution for 
is 
. Comparison of the experimental data with oxygen activities in the oxide calculated for several different defect models suggests that, for 
, any pairwise interaction between vacancies is less than 0.025 eV and the vacancies can be considered to be noninteracting. The behavior observed for 
is believed to be due to a transition to a more complex defect at high defect concentrations and not to a change in degree of ionization of the metal vacancy. The influence of carbon dissolution, from the 
mixtures used, on these measurements is discussed.