Determination of gibbs energies of formation of CaFe 2 O 4 and Ca 2 Fe 2 O 5 from solid-state EMF measurements using CaF
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een the subject of a great number of studies because of its applications to iron and steel metallurgy. At our department, a project is on-going comprising the evaluation of phase relations and thermodynamic data in the ternary system CaOFe-O. As a part of this project, Bjo¨rkman[1] investigated the subsystem CaO-Ca2Fe2O5-‘FeO’-Fe, using a galvanic cell technique incorporating calcia-stabilized zirconia (CSZ) as solid electrolyte. The standard Gibbs energy change (⌬G0) for the equilibrium reaction 2CaO (ss) ⫹ 2Fe ⫹ 3/2 O2 } Ca2Fe2O5
[1]
was evaluated, taking the solid solubility of ‘FeO’ in CaO into account. The ‘FeO’ denotes nonstoichiometric wu¨stite in equilibrium with solid iron, whereas CaO (ss) denotes CaO with a minor amount of ‘FeO’ in solid solution. The solid solubility of ‘FeO’ in CaO was found to be 5 mole pct at 1305 K. More recently, the oxygen pressure of the three-phase coexistence Ca2Fe2O5 ⫹ CaFe2O4 ⫹ Ca4Fe9O17 has been measured.[2] In order to evaluate stability data for Ca4Fe9O17, reliable thermodynamic data for Ca2Fe2O5 and CaFe2O4 are required. The phase CaFe2O4, however, is not readily amenable to study with CSZ as solid electrolyte. Phase relations in the binary subsystem CaO-Fe2O3 have been studied by Phillips and Muan[3] for oxygen pressures
of 0.21 atm (air) and 1 atm. According to their investigation, three intermediate phases Ca2Fe2O5, CaFe2O4, and CaFe4O7 are stable in this system. In air, Ca2Fe2O5 and CaFe2O4 were found to be stable up to 1722 and 1489 K, respectively, whereas the stability of CaFe4O7 is limited to the temperature range 1428 to 1499 K. Thermodynamic stability data for Ca2Fe2O5 and CaFe2O4 have been determined by Rezukhina and Baginska[4] and by Rajagopalan et al.[5,6] from solidstate emf measurements using CaF2 as the solid electrolyte. During the preparation of this manuscript, it has come to the authors’ attention that Jacob et al.[7] have determined thermodynamic data for Ca2Fe2O5 and CaFe2O4 using the same technique as Rezukhina and Baginska. The results from the different investigations using CaF2 as solid electrolyte differ considerably. The aim of the present study is to determine reliable values of the standard Gibbs energy of formation of CaFe2O4 and Ca2Fe2O5. To accomplish this, a device for electrochemical measurements using CaF2 as a solid electrolyte was constructed. Using reliable data for the Gibbs energy of formation of Fe2O3, a comparison between Bjo¨rkmans study and the present study becomes feasible, thereby receiving some information on the accuracy of the present method. As CaFe4O7 is stable only at temperatures higher than ⬃1430 K, it is not amenable for study using CaF2 as a solid electrolyte and, thus, has not been investigated. II. EXPERIMENTAL A. Materials Used and Preparations
STEFAN FORSBERG, formerly with the Department of Chemistry, Inorganic Chemistry, Umea˚ University, SE-901 87 Umea˚, Sweden, is with Studsvik Nuclear AB, SE-611 82 Nyko¨ping, Sweden. Contact e-mail: ¨ M, formerly with the [email protected] PATRIK WIKSTRO Depar
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