Variation in crystal structure of Ln2Ni1−xCuxO4+δ (Ln: La, Pr, Nd, Sm, Eu, and their solid solution) based on type of Ln: Relationship between crystal structure and tolerance factor

The variation in the crystal structure of Ln2Ni1−xCuxO4+δ, which has great potential as a new cathode material for solid oxide fuel cells, based on the type of Ln (La, Pr, Nd, Sm, Eu, and their solid solution), was investigated. For Ln with ionic radius larger than 1.21 Å (1 Å = 0.1 nm), a single-phase K2NiF4 structure called T-phase was prepared for 0.0 ≤ x ≤ 1.0. For Ln with ionic radius between 1.21 and 1.15 Å, the crystal structure changed from single T-phase to single Nd2CuO4 structure called T′-phase, through the mixture of T-phase and T′-phase, along with increasing Cu content. The range of Cu content, in which the mixture phase was obtained because of the miscibility gap, increased with the decreasing size of Ln. The single T-phase was never prepared for Ln with ionic radius below 1.15 Å. Rough tendencies were observed, indicating that the single T-phase was obtained or was not prepared if the tolerance factor was above or below 0.865, respectively. The mixture of T-phase and T′-phase was obtained because of the miscibility gap by decreasing the tolerance factor slightly below about 0.865. The T-phase was never generated by decreasing the tolerance factor any further. The change of the crystal structure by Cu content and kind of Ln can also be explained by using Jahn–Teller effect of Cu2+ ion.