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 Ln2Ni1−xCuxO4+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(O2)). For Nd2Ni1−xCuxO4+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(O2), whereas little variation was observed in ΔH and Δδ. For Nd2Ni1−xCuxO4+δ with x ≥ 0.15, crystal structure was tetragonal and no phase transition was detected below 750 °C. For Pr2Ni1−xCuxO4+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(O2) with that of Nd2Ni1−xCuxO4+δ. Δδ was not detected in TG curve of Pr2Ni0.9Cu0.1O4+δ, which could be attributed to too small Δδ. From Ellingham diagram prepared using temperature and P(O2) 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 Nd2Ni1−xCuxO4+δ and Pr2Ni1−xCuxO4+δ showed correspondence with variation of ΔH°. Excess oxygen content, δ, in Nd2Ni1−xCuxO4+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.