Construction of structural phase diagram of Nd₂Ni_{1- x}Cu_xO_{4+ δ} and effect of crystal structure and phase transition on electrical conduction behavior

The structural phase diagram of Nd₂NiO₄-Nd₂CuO₄ system, which has potential as new cathode material for solid oxide fuel cells, is established using HT-XRD and SEM-EDX. The Nd₂Ni_{1- x}Cu_xO_{4+ δ} showed single phase of orthorhombic and tetragonal K₂NiF₄-type crystal structure for 0.0 ≤ x ≤ 0.1 and 0.2 ≤ x ≤ 0.3, respectively. For the specimens with x = 0.0 and 0.1, phase transition from orthorhombic to tetragonal K₂NiF₄ phase was detected at 570 °C and 530 °C, respectively. The specimens with 0.4≤x≤0.9 were mixture of tetragonal K₂NiF₄–type structure and Nd₂CuO₄-type structure owing to the miscibility gap, which remained between 30 and 1000 °C in air. The electrical conductivity increased by increasing the Cu content from 0.0 to 0.3, which may be due to the decrease in the electrical resistivity at the grain boundary owing to the increased particle size; however, it decreased by further substitution to 0.4, which was attributed to the generation of the Nd₂CuO₄ phase by the miscibility gap.