Lattice parameter and redox equilibria in CaO-containing wustite

Equilibrium relation among oxygen potential in atmosphere, composition and lattice parameter in CaO-containing wustite have been studied at 1 273 K. Wustite plates with the molar cation fraction M-Ca/(M-Fe+M-Ca) (=MCa-Fe) ranging from 0.0058 to 0.079 were equilibrated with CO-CO2 gas mixtures. Composition of the wustite specimens was determined by chemical analysis and thermo-gravimetric method. Lattice parameter of the CaO-containing wustite was measured by X-ray diffraction method. The relation between the lattice parameter a and the gas ratio Ks (=P-CO2/P-CO) in the gas mixture was expressed by a=(4.2975+0.560.MCa-Fe)-1.470x10(-2).ln(Ks) (Angstrom) The variation of a with MCa-Fe and molar ratio M-Fe/M-O in the CaO-containing wustite was given by a=1.916+0.565.(MCa-Fe)+4.719.(M-Fe/M-O)-2.316.(M-Fe/M-O)(2) (Angstrom) Then the lattice parameter may be used as the index of the composition in CaO- containing wustite. The 'iso-lattice-parameter' line was drawn within the wustite field in the isothermal section of the system FeO-Fe2O3-CaO. The molar ratio M-Fe2O3/M-FeO of the CaO-containing wustite on the Fe2O3-rich side slightly increases with an increase in CaO content but the ratio on the FeO-rich side does not change. The solubility of CaO in the wustite equilibrated with both metallic iron and 2CaO.Fe2O3 was estimated to be similar to9 mol% CaO.