THERMODYNAMIC STUDIES ON LAFEO3(S)

The enthalpy increments and the standard molar Gibbs energies in the f ormation of LaFeO3(s) have been measured using a high-temperature Calv et micro-calorimeter and a solid oxide galvanic cell, respectively. Th e corresponding expression for enthalpy increments is given as: H degr ees(T) - H degrees(298.15 K)(J mol(-1))(+/-1.2%) = - 36887.27 + 103.53 T(K) + 25.997 x 10(-3)T(2)(K) 11.055 x 10(5)/T(K). The heat capacity, the first differential of H degrees(T) -H degrees(298.15 K) with resp ect to temperature, is given as: C(p,m)degrees(T)(JK(-1) mol(-1))= 103 .53 + 51.994 x 10(-3)T(K) - 11.055 X 10(5)/T-2(K). From the measured e .m.f. of the cell, (-)Pt/(LaFeO3(s) + La2O3 (s)+ Fe(s))//CSZ//(Ni(s) NiO(s))/Pt(+), and the relevant Delta(f)G(m)degrees(T) values from th e literature, the Delta(f)G(m)degrees(LaFeO3, s, T) was calculated, an d is given as: Delta(f)G(m)degrees(LaFeO3, s, T)(kTmol(-1))(+/-0.72) = - 1319.2 + 0.2317T(K).The calculated Delta(f)H(m)degrees(LaFeO3, s, 2 98.15 K) and S degrees(298.15 K) values obtained using the second law method are -1334.7 kJ mol(-1) and 128.9 J K-1 mol(-1), respectively. ( C) 1998 Elsevier Science S.A. All rights reserved.