Thermodynamic analyses of the orthorhombic-to-tetragonal phase transition in Pr_2−xNd_xNiO_4+δunder controlled oxygen partial pressures

The behavior of the structural orthorhombic-tetragonal phase transition of Pr_2−xNd_xNiO_4+δ, a candidate material for solid oxide fuel cells and oxygen permeation membranes, was investigated by differential scanning calorimetry and thermogravimetry (TG), under controlled oxygen partial pressures,P(O₂). The structural phase transition temperature,T_P, of Pr_2−xNd_xNiO_4+δincreased with increasing Nd content,x, or increasingP(O₂). The phase transitions of all compositions involved discrete variations in the oxygen content, Δδ, which were observed in the TG curves under variousP(O₂) values. Δδof Pr_2−xNd_xNiO_4+δwith 0.5 ≤x≤ 1.5 were between those of Nd₂NiO_4+δand Pr₂NiO_4+δ, regardless ofP(O₂), and were slightly increased with decreasingP(O₂). It was proposed that the effect of the valence change of the Pr ion on Δδwas decreased with increasing Nd content. The standard enthalpy change, ΔH°, and entropy change, ΔS°, of the phase transition were estimated from the Ellingham diagrams and van't Hoff plots, which were prepared from the relationship betweenP(O₂) andT_Pusing an ideal solution model. ΔS° was decreased with increasing Nd content for the specimens with 0.0 ≤x≤ 1.5. The ΔH° of Pr_2−xNd_xNiO_4+δwith 0.0 ≤x≤ 1.5 was almost constant for all Nd contents. The increase in the phase transition temperature of Pr_2−xNd_xNiO_4+δwith increasingxfrom 0.0 to 1.5 was successfully explained using the calculated values of ΔH° and ΔS°.