Nonstoichiometry (δ) and High-Temperature Thermodynamic Properties of (Mg0.22Mn0.07Fe0.71)3−δO4 Ferrite Spinel

Abstract

Nonstoichiometry of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite has been measured in the temperature range of 1000×1200°C as a function of oxygen activity (aO2) using a solid state coulometric titration technique. At a given temperature, the nonstoichiometry (δ) was found to vary with the oxygen activity as δ=[V]0 · a2/3O2 −[I]0·a−2/3O2 where [V]0 and [I]0 are constants. This aO2-dependence of δ was explained in terms of Frenkel disorder, i.e., cation vacancy and interstitial. From the temperature dependence of the defect constants ([V]0 and [I]0) and the oxygen activity for the stoichiometric composition (δ=0), thermodynamic quantities such as enthalpies for the defect formation reactions and relative partial molar enthalpy and entropy of oxygen were calculated.