Standard enthalpy of formation and heat capacity of compounds in the pseudo-binary Bi2O3-Fe2O3 system

Bismuth ferrites, Bi2Fe4O9, BiFeO3, and Bi25FeO39 have been prepared and characterized. The thermodynamic data such as standard molar enthalpies of formation Delta H-f(m)degrees(298.15 K) and standard molar heat capacity (C-p(degrees)) were obtained from the solution calorimetry and calvet calorimetric measurements. Delta H-f(m)degrees (298.15 K) for Bi2Fe4O9(s), BiFeO3(s), and Bi25FeO39(s) were found to be (-2476.0 +/- 4.3), (-768.4 +/- 2.9) and (-7699.8 +/- 17.3) kJ mol(-1), respectively. The dependence of molar heat capacity C-p degrees with temperature can be given as C-p(degrees)(Bi2Fe4O9, s) (J K-1 mol(-1)) = 353.81 + 0.01774T - 1.63606 x 10(5)/T-2 (313 < T/K < 911) C-p(degrees)(BiFeO3, s) (J K-1 mol(-1)) = 116.72 + 0.00968T - 1.9273 x 10(5)/T-2 (313 < T/K < 911) and C-p(degrees)(Bi25FeO39, s) (J K(-1)mol(-1)) = 1339.21 + 0.16550T - 28.3546 x 10(5)/T-2 (324 < T/K < 834). The standard Gibbs energy of formation of the above compounds have been derived using the values of Delta fH(m)degrees (298.15 K), C-p(degrees) and the estimated Delta fS(m)(degrees) (298.15 K). (c) 2007 Elsevier B.V. All rights reserved.