Abstract
The orthorhombic-tetragonal structural phase transition behaviors of Pr2NiO4+δ and Nd2NiO4+δ were investigated by DSC and TG–DTA under controlled oxygen partial pressure, P(O2), and their difference was discussed. Endothermic peak corresponding to the structural phase transition was observed in DSC curves. The transition temperature, Tp, decreases with decreasing P(O2) both in Pr2NiO4+δ and Nd2NiO4+δ. A discrete weight variation originating from the variation of oxygen content, Δδ, was observed during the phase transition both in Pr2NiO4+δ and Nd2NiO4+δ. The Δδ of Pr2NiO4+δ was larger than that of Nd2NiO4+δ, and the Δδ of Pr2NiO4+δ increased with decreasing P(O2), while that of Nd2NiO4+δ was independent on P(O2), suggesting that not only the valence of Ni but also that of Pr changed during the phase transition in Pr2NiO4+δ. From the Ellingham diagram prepared using Tp measured by DSC under various P(O2), ΔH° and ΔS° at the phase transition were evaluated. It was revealed that ΔH° of Nd2NiO4+δ is larger than that of Pr2NiO4+δ, whereas ΔS° shows little difference, showing agreement with higher Tp of Nd2NiO4+δ that that of Pr2NiO4+δ.
https://ror.org/0112mx960 
