Gibbs

Determination of Gibbs Energies of Formation of CaFe2O4 and Ca2Fe2O5 from Solid-State EMF Measurements Using CaF2 as Solid Electrolyte
´ ¨ STEFAN FORSBERG, PATRIK WIKSTROM, and ERIK ROSEN The standard Gibbs energy change for the reactions CaO Fe2O3 → CaFe2O4 and 2 CaO Fe2O3 → Ca2Fe2O5 has been determined from solid-state electromotive force (emf) measurements, using CaF2 as solid electrolyte.The temperature dependence of the standard Gibbs energies of formation from the elements is represented by the equations
fG 0

(CaFe2O4)

1448.4

103 1030 103 1140

128.0 T T /K 12.84 T T /K

27.95 T ln (T ) ( 1800) J mol 1280 55.91 T ln (T ) ( 2800) J mol 1340

1

fG

0

(Ca2Fe2O5)

2049.9

1

By applying a “third-law” analysis of the experimental data, the standardenthalpy of formation of 17.5 0.4 kJ CaFe2O4 from constituent oxides at 298 K was evaluated as H 0298(CaFe2O4) mol 1. A description of the developed apparatus for emf measurements is given.

I. INTRODUCTION

THE system CaO-Fe-O has for several decades been the subject of a great number of studies because of its applications to iron and steel metallurgy. At our department, a project is on-goingcomprising the evaluation of phase relations and thermodynamic data in the ternary system CaOFe-O. As a part of this project, Bjorkman[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] wasevaluated, taking the solid solubility of ‘FeO’ in CaO into account. The ‘FeO’ denotes nonstoichiometric wustite ¨ 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 beenmeasured.[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] fromsolidstate 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 presentstudy 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 Bjorkmans study and ¨ the present study becomes feasible, thereby receiving some information on theaccuracy 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...