Thermal analysis of structural phase transition behavior of Ln ₂ Ni _1−x Cu _x O _4+δ (Ln = Nd, Pr) under various oxygen partial pressures

Structural phase transition behavior between orthorhombic and tetragonal in Ln ₂ Ni _1−x Cu _x O _4+δ (Ln = Nd, Pr), which attracts interest as new cathode material for solid oxide fuel cells, has been investigated with thermal analysis under controlled oxygen partial pressures (P(O ₂ )). For Nd ₂ Ni _1−x Cu _x O _4+δ with 0.0 ≤ x ≤ 0.1, temperature, enthalpy change (ΔH) and change in excess oxygen content (Δδ) at the phase transition decreased with increasing Cu content. The phase transition temperature decreased with decreasing P(O ₂ ), whereas little variation was observed in ΔH and Δδ. For Nd ₂ Ni _1−x Cu _x O _4+δ with x ≥ 0.15, crystal structure was tetragonal and no phase transition was detected below 750 °C. For Pr ₂ Ni _1−x Cu _x O _4+δ with x = 0.0 and 0.1, phase transition was observed by DSC, showing similar dependence of phase transition temperature and ΔH on Cu content and P(O ₂ ) with that of Nd ₂ Ni _1−x Cu _x O _4+δ . Δδ was not detected in TG curve of Pr ₂ Ni _0.9 Cu _0.1 O _4+δ , which could be attributed to too small Δδ. From Ellingham diagram prepared using temperature and P(O ₂ ) at the phase transition, variation of standard enthalpy (ΔH°) and standard entropy (ΔS°) was evaluated. It was revealed that variation of phase transition temperature by Cu content and difference of the phase transition temperature between Nd ₂ Ni _1−x Cu _x O _4+δ and Pr ₂ Ni _1−x Cu _x O _4+δ showed correspondence with variation of ΔH°. Excess oxygen content, δ, in Nd ₂ Ni _1−x Cu _x O _4+δ , evaluated with reduction in TG apparatus, decreased with increasing Cu content. It was suggested that some amount of δ was required for stabilization of orthorhombic phase and that low δ by Cu substitution stabilized the tetragonal structure of the specimens with x ≥ 0.15.