Analysis of thermal stability of LaNi_1-xFe_xO_3-δ (x = 0.0, 0.2, 0.4) by thermogravimetry and high-temperature X-ray diffraction under controlled oxygen partial pressures

Thermodynamic stability of LaNi_1-xFe_xO_3-δ (x = 0.0, 0.2, 0.4), which is expected as new cathode material for solid oxide fuel cells, was investigated by thermogravimetry and high-temperature X-ray diffraction under controlled oxygen partial pressure, P(O₂). It was clarified that LaNiO_3-δ decomposed through La₄Ni₃O₁₀ to La₂NiO₄ with increasing temperature under P(O₂) of 10^-2 atm or more. The decomposition temperature decreased with reducing P(O₂). Under P(O₂) of 10^-3 atm, LaNiO_3-δ directly decomposed to La₂NiO₄. The decomposition of LaNi_1-xFe_xO_3-δ (x = 0.2, 0.4) to La₄(Ni_1-xFe_x)₃O₁₀ was also observed, and Ellingham diagram of the decomposition reaction was prepared. It was revealed that thermodynamically stable temperature and P(O₂) region expanded with increasing Fe content. Nonlinear decomposition boundary was observed in the Ellingham diagram of LaNiO_3-δ and LaNi_0.8Fe_0.2O_3-δ whose origin can be suspected to the difference of δ at the decomposition on P(O₂). Linear decomposition boundary was observed for LaNi_0.6Fe_0.4O_3-δ, indicating variation of entropy, ΔS°, and enthalpy, ΔH°, at the decomposition were 151 J mol^-1 K^-1 and 240 kJ mol^-1, respectively.