Ln₂NiO_4+δ (Ln=La, Pr, Nd) with K₂NiF₄ structure (T-phase) has attracted much interest as new cathode material for solid oxide fuel cells. It was reported that partial substitution of Cu for Ni-site in Pr₂NiO_4+δ improved ionic and hole conductivity [1]. For development of superior cathode material, Cu content should be optimized. It is expected that crystal structure must change depending on Cu content, resulting in possible variation of electrical property, since crystal structure of Ln₂CuO₄ (Ln=Pr, Nd) is T’-phase, which is different from T-phase. In the former presentation, structural phase diagram of Nd₂Ni_1-xCu_xO_4+δwith existence of miscibility gap has been reported. Since the crystal structure and electrical property also depend on kinds of Ln, as reported by Miyoshi and coworkers [1], optimization of not only Cu content but also kinds of Ln are also required.

In this study, preparation of Ln₂Ni_1-xCu_xO_4+δ (Ln=La, Pr, Nd, Sm, Eu) with various compositions was attempted and stability at high temperature was evaluated to establish structural phase diagram. In particular, variation of miscibility gap region on kinds of Ln was investigated.

Ln₂Ni_1-xCu_xO_4+δ (Ln=La, Pr, Nd, Sm, Eu) was prepared by the Pechini method by which preparation of homogenous specimen can be expected. La₂O₃, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, Eu₂O₃ and  CuO powder were dissolved with mixture of dilute HNO₃ and H₂O₂. Ni(NO₃)₂·6H₂O powder was dissolved with distilled H₂O. They were mixed with nominal composition. After addition of ethylene glycol and citric acid, the mixture was heated at about 450 ºC, resulting in precursor. The obtained precursor was calcined at 700 ºC for 24 h in air, followed by pressing into pellet and heat-treatment at 1000-1200 ºC in air for 10 h.

For identification of the crystal structure and evaluation of the lattice constants, X-ray diffraction measurements (XRD) were performed. Some XRD patterns were analyzed using Rietveld analysis employing program RIETAN-FP [2]. Homogeneity of the specimens was evaluated by SEM-EDX analysis (JEOL: JCM-5700 equipped with JED-2300). Temperature dependence of crystal structure and stability at high temperature were evaluated using high-temperature XRD in air.

XRD patterns at room temperature of La₂Ni_1-xCu_xO_4+δ prepared at 1200 ºC could be indexed as single tetragonal T-phase and single orthorhombic T-phase for 0.1≤x≤0.8 and 0.9≤x≤1.0, respectively.

Pr₂Ni_1-xCu_xO_4+δ with 0.0≤x≤0.4 prepared at 1200 ºC could be identified as single orthorhombic T-phase at room temperature. XRD patterns at room temperature of Nd₂Ni_1-xCu_xO_4+δ prepared at 1200 ºC could be indexed as single orthorhombic T-phase and single tetragonal T-phase for 0.0≤x≤0.1 and 0.2≤x≤0.3, respectively. For diffraction patterns of 0.5≤x≤0.95 in Pr₂Ni_1-xCu_xO_4+δ and 0.4≤x≤0.9 in Nd₂Ni_1-xCu_xO_4+δ, peaks assigned as T’-phase were observed in addition to ones indexed as T-phase. With increasing Cu content, intensity of the signals indexed as T’-phase increased and ones assigned as T-phase decreased. All the peaks of XRD pattern for the specimens with x=1.0, i. e., Pr₂CuO₄ and Nd₂CuO₄ were successfully indexed as tetragonal T’-phase.

Ln₂Ni_1-xCu_xO_4+δ (Ln=Sm, Eu) for 0.95≤x≤1.0 prepared at 1000 ºC could be indexed as single tetragonal T’-phase. Contrary to Ln₂Ni_1-xCu_xO_4+δ with Ln=La, Pr, Nd, T-phase was never observed for the specimens with xless than 0.95.

Rietveld analysis revealed that the lattice constants and molar volume of both T-phase and T’-phase in the specimens with 0.5≤x≤0.95 in Pr₂Ni_1-xCu_xO_4+δ and 0.4≤x≤0.9 in Nd₂Ni_1-xCu_xO_4+δ were independent on Cu content, indicating existence of miscibility gap for 0.5≤x≤0.95 in Pr₂Ni_1-xCu_xO_4+δ and 0.4≤x≤0.9 in Nd₂Ni_1-xCu_xO_4+δ. Also revealed by Rietveld analysis was linear relationship between Cu content and molar ratio of T’-phase in the both miscibility gaps, which corresponded to lever rule.

EDX image and spectra of Pr₂Ni_1-xCu_xO_4+δ with 0.5≤x≤0.95 and Nd₂Ni_1-xCu_xO_4+δ with 0.4≤x≤0.9 indicated that the specimens were composed of two phases with Ni rich and Ni poor composition and that ratio of Ni and Cu in both phase was almost constant irrespective of x, showing agreement with the proposed miscibility gap by Rietveld analysis.

Fig. 1 shows relationship between tolerance factor and Cu content in Ln₂Ni_1-xCu_xO_4+δ. The miscibility gap was roughly observed for tolerance factor between 0.855 and 0.865, suggesting that ionic radius of Ln mainly affected crystal structure of Ln₂Ni_1-xCu_xO_4+δ.

High temperature XRD measurements revealed that the miscibility gap for 0.5≤x≤0.95 in Pr₂Ni_1-xCu_xO_4+δ was maintained up to 700 ºC. Above 800 ºC, decomposition was observed. Also revealed from high temperature XRD that the miscibility gap for 0.4≤x≤0.9 in Nd₂Ni_1-xCu_xO_4+δwas maintained up to 1000 ºC.

References

[1] S. Miyoshi et al., J. Electrochem. Soc., 154(2007) B58.

[2] F. Izumi and K. Momma, Solid State Phenom., 130(2007) 15.