Thermodynamic analyses of structural phase transition of Pr2NiO4+δ involving variation of oxygen content

Structural phase transition behavior of Pr2NiO4+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 Pr2NiO4+δ and that the phase transition temperature, Tp, decreased with decreasing P(O2). Variation of enthalpy, ΔH, and entropy, ΔS, at the phase transition per one molar Pr2NiO4+δ were calculated from the peak area of DSC and they were independent on P(O2) between 1.0 × 10−1 bar and 2.25 × 10−3 bar. From the linear relationships between Tp and RTpln P(O2) and between 1/Tp and Rln P(O2), 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 Pr2NiO4+δ by P(O2).