High Temperature Oxidation, Range of Non-Stoichiometry and Curie Point Variation of Cation Deficient Titanomagnetite Fe2.4Ti0.6O4+

Synthetic spinel tita omagnetite of compositia Fe2.4Ti0.6O4+γ was equilibrated with CO-CO2 mixtures at 1275° to produce a series of samples with varying oxidation parameter γ within the monophasic stability field. After equilibrium had been attained all samples were sharply quenched in the same way. Curie temperature (Tc) was measured and the parameter γ was determined from weight changes to an accuracy of 0.01 per cent by means of Cahn Gram Electrobalance. A very strong practically linear dependence of Tc upon oxidation parameter has been found: Tc(γ) = Tc(γ0)+890γ; (0 < γ < 0.06). The slope, 890 (Kγ-1) appears to be considerably steeper, namely by a factor of 9, than the Tc(γ) dependence for a titanomagnetite of the same composition oxidized to its maghemite correlate, i.e. below about 350 °C. The relatively high sensitivity, reversibility and reproducibility with which the above relation is obeyed is good enough to utilize Tc as a diagnostic parameter for the degree of non-stoichiometric imbalance in titanomagnetites which were rapidly cooled, provided the iron-titanium ratio is known. This finding may explain the large scatter of values of Tc so far published (up to 60 K for Fe2.4Ti0.6O4).