Thermal Behavior and Cation Distribution of Submicron Copper Ferrite Spinels CuxFe3−xO4(0 ≤x≤ 0.5) Studied by DTG, FTIR, and XPS

Abstract

The valence state and the cation distribution on octahedral (B) and tetrahedral (A) sites in the spinel structure of Cu and Fe ions has been determined in nanometric copper-substituted magnetites, CuxFe3−xO4(0 ≤x≤ 0.5), by TG, DTG, FTIR, and XPS. The oxidation of the oxidizable cations has also been investigated: It was demonstrated that Cu+B, Fe2+B, and Cu+Aoxidize into Cu2+and Fe3+ions below 300°C. Moreover, the stability of each cation toward oxidation, decreasing from Cu+B to Cu+A, has been found in close relation with the cation–oxygen distance of each oxidizable cation. A quantitative analysis by DTG based on this difference of reactivity only permits one to determine the concentration of B-site (Fe2++ Cu+) ions and A-site Cu+ions. Using TG analysis performed on different experiments varying the oxygen partial pressure, the cationic distribution has been obtained. It is given by the formula (Cu+αxFe3+1−αx)A(Cu3+(1−α−β)xCu+βxFe3+1+(2α+β)xFe2+1−(1+α+β)x)BO2−4, where α and β varies withx. α = 0.50 and β = 0.20 forx= 0.32. From FTIR and XPS data, it has been shown that when the deviation from stoichiometry is the greatest the vacancies and all the Cu2+ions are on B-sites.