Thermodynamic analyses of structural phase transition of Pr ₂NiO_4+δ involving variation of oxygen content

Structural phase transition behavior of Pr₂NiO _4+δ, which attracts interest as new cathode material for solid oxide fuel cells, was investigated by DSC and TG-DTA under controlled oxygen partial pressure, P(O₂). It was revealed that the structural phase transition from orthorhombic phase to tetragonal phase involved discrete decrease of oxygen content in Pr₂NiO_4+δ and that the phase transition temperature, T_p, decreased with decreasing P(O ₂). Variation of enthalpy, ΔH, and entropy, ΔS, at the phase transition per one molar Pr₂NiO_4+δ were calculated from the peak area of DSC and they were independent on P(O ₂) between 1.0 × 10^-1 bar and 2.25 × 10 ^-3 bar. From the linear relationships between T_p and RT_pln P(O₂) and between 1/T_p and Rln P(O ₂), variation of standard enthalpy, ΔH, and standard entropy, ΔS, of the phase transition per one molar O₂ were calculated, showing fair agreement with ΔH and ΔS. The variation of oxygen content at the phase transition, Δδ, increased with decreasing P(O₂), which can be attributed to valence variation of Pr in tetragonal Pr₂NiO_4+δ by P(O₂).