Preparation of Structural Phase Diagram of Ln_2Ni_(1-x)Cu_xO_(4+δ) (Ln=La, Pr, Nd, Sm, Eu) as New Cathode Materials: Variation of Structural Phase Diagram on Kind of Ln

摘要

Crystal structure of Ln_2Ni_(1-x)Cu_xO_(4+δ) (Ln= La, Pr, Nd, Sm and Eu), which are expected as new cathode materials for solid fuel cells, was investigated. Single phase with K_2NiF_4 structure, so-called T-phase, was obtained for La_2Ni_(1-x)Cu_xO_(4+δ) regardless of Cu content. Single phase with Nd_2CuO_4 structure, so-called T'-phase, was obtained for Sm_2Ni_(1-x)Cu_xO_(4+δ) and Eu_2Ni_(1-x)Cu_xO_(4+δ) for only 0.95≤x≤1.0 and x=1.0 respectively. No T-phase was obtained for the other composition of Ln_2Ni_(1-x)Cu_xO_(4+δ) (Ln=Sm, Eu). The single T-phase and T'-phase were obtained for Pr_2Ni_(1-x)Cu_xO_(4+δ) with x≤0.4 and x=1.0, respectively. For Pr_2Ni_(1-x)Cu_xO_(4+δ) with x between 0.5 and 0.9, phase separation into T-phase and T'-phase due to miscibility gap was observed, showing similar behavior with Nd_2Ni_(1-x)Cu_xO_(4+δ) except for little smaller x range of the miscibility gap. From high temperature X-ray diffraction measurements, it was revealed that the miscibility gap between 0.5 and 0.9 of Pr_2Ni_(1-x)Cu_xO_(4+δ) remained up to 700°C. Variation of the crystal structure of Ln_2Ni_(1-x)Cu_xO_(4+δ) on kinds of Ln and Cu content could be explained by using tolerance factor calculated from ionic radius of Ln~(3+), Ni~(2+) and Cu~(2+).