Evaluation of vacancies and interstitials by thermogravimetry

A procedure to determine the vacancy concentration [V] in magnetite from TG curves when heating alpha-Fe2O3 in a reducing atmosphere is presented, tog ether with some experimental results. The procedure correlates vacancy and cation concentration with the weight change due to oxygen evolution through the charge and mass balance in the sample. The results are presented toget her with a generalized summary of previous findings on mixed spinel ferrite s and yttrium iron garnet. It is shown that in all cases the procedure lead s to the same general expression theta = Gamma mu(B)/mu(o2) (1 - m(f)/m(i)) where m(f) and m(i) are the final and initial mass of the sample, mu(B) the formula mass of the initial oxide reagents, mu(o2) the oxygen molar mass a nd theta a parameter associated with the oxygen evolution. The parameter Ga mma, together with the explicit dependence [V](theta) (or interstitial conc entration l(theta) in the case of YIG) must be determined in every case fro m the specific chemical equations. The Gamma values derived for different f errite materials are tabulated together with the corresponding [V](theta) o r [I](theta). The procedure allows the calculation of the Fe3+-->Fe2+ entha lpy from the equilibrium In(K) versus 1/T curves deduced from the TG data a nd the Van't Hoff equation. (C) 2000 Elsevier Science B.V. All rights reser ved.