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A-site deficient Ruddlesden–Popper phases now attract a lot of attention as promising materials for intermediate temperature solid oxide fuel cells' cathodes and oxygen separation membranes. However, controversial information on the A-site deficiency effect on structural, transport and electrochemical properties of these materials apparently requires further studies of such systems. In this work, structural and transport features of (Nd2-xCax)0.975NiO4+δ system were studied. (Nd2-xCax)0.975NiO4+δ (x = 0–0.4) samples were synthesized via a solution-assisted solid state reaction method and characterized by X-ray diffraction. Oxygen transport was studied by the temperature programmed isotope exchange of oxygen with C18O2 in the flow reactor. A high oxygen mobility of materials (D* ~ 10−7 cm2/s at 700 °C) is provided by the cooperative mechanism of oxygen migration involving both regular and highly-mobile interstitial oxygen. As typical for Ruddlesden–Popper phases, doping with Ca leads to increase of total conductivity due to holes' formation and decrease of oxygen mobility due to hampering cooperative mechanism, however, this effect is less pronounced compared to A-site stoichiometric Nd nickelates which can be attributed to oxygen vacancies formation and their participation in cooperative migration as well as to other features of A-site deficiency. High mixed ionic-electronic conductivity of materials makes them promising in using as solid oxide fuel cells’ cathodes and oxygen separation membranes.

One of the main directions for the attainment of high-performance solid oxide electrochemical cells, including those based on proton-conducting electrolytes, consists in the rational engineering of functional materials. Herein, we examine the use of Nd2–xBaxNiO4+δ (NBNx, 0 ≤ x ≤ 0.4) materials as potential oxygen electrodes for protonic ceramic fuel cells (PCFCs) based on conventional Ba(Ce,Zr)O3-electrolytes. A Ba-doping strategy is proposed for Nd2NiO4+δ for reducing chemical interdiffusion of barium-ions from electrolyte to electrode during long-term operation. It is experimentally confirmed that a moderate Ba-content is beneficial in terms of reducing the quantity of impurity phases formed at the NBNx/Ba(Ce,Zr)O3 interface during deliberately-imposed extreme temperature conditions (1250 °C for 20 h). In addition, materials with x = 0.1 and 0.2 exhibit the highest electrical conductivity with no adverse effects on their thermomechanical properties. Thus, when characterising symmetrical cells in wet air conditions, NBN0.1 and NBN0.2 electrodes demonstrates lower polarisation resistances (down to 1.7 Ω cm2 at 700 °C) compared with NBN0, NBN0.3 and NBN0.4 electrodes. Based on these findings, the Ba-doping of nickelates can be recommended as a promising modification for the design of electrodes having good performance and chemical compatibility.

Although the correlated rare-earth nickelate perovskites (such as SmNiO3) exhibit remarkable metal to insulator transitions (MIT) beyond conventional semiconductors, their respective variations in mechanical properties during MIT remain yet unknown. Exploring the intrinsic mechanical properties of SmNiO3 is largely prohibited since it is theoretically impossible to grow the single crystalline samples in a large size as required for conventional characterizations. For the first time, we performed nano-indentation tests of preferentially oriented SmNiO3 films with thickness approaching micrometer scale epitaxy on single crystalline LaAlO3 substrates, and the corresponding temperature-dependent mechanical properties are characterized. As-grown SmNiO3 thick films experience sharp transitions in electrical resistivity across MIT, while their insulating phases exhibit thermistor transportation behaviors with the negative temperature dependence of resistance within a broad range of temperature. Nano-indentation tests at room and elevated temperature regions (25–200 °C) are performed for harvesting the Vickers hardness and elastic modulus at different temperatures. In addition, it is also interesting to note that these mechanical properties remain relatively stable during MIT, despite the structural twisting in their NiO6 octahedron during the MIT as previously known for SmNiO3. The SmNiO3 is shown to be relatively soft among the family of perovskite oxides, while its mechanical properties are insensitive to increase temperature across its metal to insulator transitions. It paves the way for further applying rare-earth nickelates in the field of correlation electronics.