CRYSTAL STRUCTURE AND THERMODYNAMIC STABILITY OF PHASES IN THE (La,Pr,Nd)₂(Ni,Cu)O₄

Sukhanov K.S., Gilev A.R., Kiselev E.A., Cherepanov V.A.

Ural Federal University

620002, Ekaterinburg, Mira st., 19

The Ruddlesden-Popper Ln₂NiO_4+δ (Ln=La, Nd, Pr) complex oxides are considered by researchers as promising materials for electrochemical devices, particularly, as cathodes for intermediate-temperature (600-800 °C) solid oxide fuel cells (IT-SOFCs). Insufficient thermodynamic stability at T<900 °C in air is their main disadvantage. Stability can be improved by partial replacing of praseodymium with lanthanum/neodymium and nickel with copper. The purpose of this work is to synthesize and to study the stability of La_2-x(Pr,Nd)_xNi_1-yCu_yO_4+δ at 700 °C in air.

The La_2-x(Pr,Nd)_xNi_1-yCu_yO_4+δ (x = 0,5; 1,0; 1,5; y = 0,4; 0,6; 0,8) complex oxides were synthesized via a citrate-nitrate process. The phase purity of the samples was confirmed by XRD analysis. The XRD results showed that La_2-x(Pr,Nd)_xNi_1-yCu_yO_4+δ (x=0,5–1,0; y=0,4–0,6) and La_0.5Pr_1.5Ni_0.6Cu_0.4O_4+δ complex oxides were single-phase after annealing at 900 °C with the K₂NiF₄-type tetragonal structure. The La_1.5Pr_0.5Ni_0.2Cu_0.8O_4+δ and La_1.5Nd_0.5Ni_0.2Cu_0.8O_4+δ samples contained little amounts (< 0.6%) of PrO_x and Nd₂O₃. Impurities were found in the remaining samples, the content of which increased with increasing concentrations of praseodymium/neodymium and copper. For single-phase samples, a parameter decreases as the dopant concentration increases. The c parameter increases with copper doping and decreases with increasing concentration of praseodymium/neodymium. Structural stability with the addition of dopants can be described using the tolerance factor of the Paul Poix (t) for phases with the K₂NiF₄ type structure [1, 2]. Limiting value of t=0.85 could define single phase domains of the studied solutions satisfactorily. To study the thermodynamic stability in the intermediate-temperature range (600-800 °C), single-phase oxides obtained at 900 °C were kept at 700 °C in air for 30 days. The XRD results showed that all solid solutionsLa_2-xNd_xNi_1-yCu_yO_4+δ were stable after annealing at 700 °C in air. In the La_2-xPr_xNi_1-yCu_yO_4+δ system, compositions with x = 0.5 and y = 0.6; 0.8 were stable at 700 °C. Other studied solid solutions of La_2-xPr_xNi_1-yCu_yO_4+δ contained significant amounts (> 5%) of La_1-xPr_xNi_1-yCu_yO_3-δ and PrO_x impurity phases.

1. Poix P. Etude de la structure K₂NiF₄ par la methode des invariants, I. Cas des oxydes A₂BO₄ // J. Solid State Chem. – 1980. – V. 31. – P. 95–102. https://doi.org/10.1016/0022-4596(80)90011-0

2. Ganguly P., Rao C.N.R. Crystal Chemistry and Magnetic Properties of Layered Metal Oxides Possessing the K₂NiF₄ or Related Structures // J. Solid State Chem. – 1984. – V. 53. – P. 193–216. https://doi.org/10.1016/0022-4596(84)90094-X

This work was carried out with the financial support of the Ministry of Science and Higher Education of the Russian Federation (theme № FEUZ-2026-0011).