Preparation of Structural Phase Diagram of Ln2Ni1-X
Cu
x
O4+δ
(Ln=La, Pr, Nd, Sm, Eu) as New Cathode Materials: Variation of Structural Phase Diagram on Kinds of Ln
Authors: Soga Haruki , Wang Chengkun , Hayashi Tomoki , Morise Takahiko , Niwa Eiki , Hashimoto Takuya 
Published: February 2020

Ln2NiO4+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 Pr2NiO4+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 Nd2Ni1-x
Cu
x
O4+δ
with 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 Ln2Ni1-x
Cu
x
O4+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.

Ln2Ni1-x
Cu
x
O4+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 La2Ni1-x
Cu
x
O4+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.

Pr2Ni1-x
Cu
x
O4+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 Nd2Ni1-x
Cu
x
O4+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 Pr2Ni1-x
Cu
x
O4+δ
and 0.4≤x≤0.9 in Nd2Ni1-x
Cu
x
O4+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.

Ln2Ni1-x
Cu
x
O4+δ
(Ln=Sm, Eu) for 0.95≤x≤1.0 prepared at 1000 ºC could be indexed as single tetragonal T’-phase. Contrary to Ln2Ni1-xCuxO4+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 Pr2Ni1-x
Cu
x
O4+δ
and 0.4≤x≤0.9 in Nd2Ni1-x
Cu
x
O4+δ
were independent on Cu content, indicating existence of miscibility gap for 0.5≤x≤0.95 in Pr2Ni1-x
Cu
x
O4+δ
and 0.4≤x≤0.9 in Nd2Ni1-x
Cu
x
O4+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 Pr2Ni1-x
Cu
x
O4+δ
with 0.5≤x≤0.95 and Nd2Ni1-x
Cu
x
O4+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 Ln2Ni1-x
Cu
x
O4+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 Ln2Ni1-x
Cu
x
O4+δ
.

High temperature XRD measurements revealed that the miscibility gap for 0.5≤x≤0.95 in Pr2Ni1-x
Cu
x
O4+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 Nd2Ni1-x
Cu
x
O4+δ
was 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.

Figure 1