Thermodynamic properties of iron oxides and hydroxides. II. Estimation of the surface and bulk thermodynamic properties of ordered and disordered maghemite (γ-Fe203)
Diakonov, Igor I.
published: Jan 26, 1998
manuscript accepted: Sep 1, 1997
manuscript received: Sep 30, 1996
ArtNo. ESP147051001003, Price: 29.00 €
Abstract A consistent set of the thermodynamic properties of ordered and disordered maghemite (γ-Fe2O3) was generated from literature data by accounting for the effect of surface thermodynamic properties. The experimental high-temperature heat-capacity measurements for ordered and disordered maghemite are consistent with the equations: Cp° = 87.84 + 0.07518T (300-700 K) and Cp° = 52.94 + 0.1713T (300-600 K), respectively. The standard enthalpy of formation of ordered and disordered maghemite at 298.15 K (ΔfH° = -805.8 ±5.7 kJ/mol) was generated from the measured enthalpy of transition of maghemite to hematite, using the apparent surface enthalpy of maghemite derived from the measured enthalpy of solution of this mineral as a function of surface area. The surface Gibbs energy of maghemite (GA = 690 ± 200 mJ/m2) was calculated from the experimental surface enthalpy and estimated surface entropy. In the absence of low-temperature heat-capacity data and because of large scattering of estimated values, the standard entropy of ordered maghemite at 298.15 K was taken to be that of hematite (S° = 87.4 J/mol K, Hemingway, 1990). Consequently, the standard entropy of disordered maghemite was calculated as 91.4 J/mol K. Using the standard entropies and enthalpy of formation, the standard Gibbs energies of formation at 298.15 K were derived as ΔfG° = -723.9±6.4 and ΔfG° = -725.1 ±6.4kJ/mol for ordered and disordered maghemite, respectively. Stability of maghemite with respect to hematite was examined for temperatures up to 1150 K.