Thermodynamic analyses of the orthorhombic-to-tetragonal phase transition in Pr2-xNdxNiO4+delta under controlled oxygen partial pressures

摘要

The behavior of the structural orthorhombic-tetragonal phase transition of Pr2-xNdxNiO4+delta, 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(O-2). The structural phase transition temperature, T-P, of Pr2-xNdxNiO4+delta increased with increasing Nd content, x, or increasing P(O-2). The phase transitions of all compositions involved discrete variations in the oxygen content, Delta delta, which were observed in the TG curves under various P(O-2) values. Delta delta of Pr2-xNdxNiO4+delta with 0.5 <= x <= 1.5 were between those of Nd2NiO4+delta and Pr2NiO4+delta, regardless of P(O-2), and were slightly increased with decreasing P(O-2). It was proposed that the effect of the valence change of the Pr ion on Delta delta was decreased with increasing Nd content. The standard enthalpy change, Delta H degrees, and entropy change, Delta S degrees, of the phase transition were estimated from the Ellingham diagrams and van't Hoff plots, which were prepared from the relationship between P(O-2) and T-P using an ideal solution model. Delta S degrees was decreased with increasing Nd content for the specimens with 0.0 <= x <= 1.5. The Delta H degrees of Pr2-xNdxNiO4+delta with 0.0 <= x <= 1.5 was almost constant for all Nd contents. The increase in the phase transition temperature of Pr2-xNdxNiO4+delta with increasing x from 0.0 to 1.5 was successfully explained using the calculated values of Delta H degrees and Delta S degrees.