Song, J.; Ning, D.; Boukamp, B.; Bassat, J.M.; Bouwmeester, H.J.M.: Structure, electrical conductivity and oxygen transport properties of Ruddlesden-Popper phases Ln_{n+1}Ni_nO_{3n+1} (Ln = La, Pr and Nd; n=1, 2 and 3). Journal of Materials Chemistry A 8 (2020), p. 22206-22221
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Layered Ruddlesden–Popper (RP) lanthanide nickelates, Lnn+1NinO3n+1(Ln¼La, Pr and Nd;n¼1, 2 and 3),are considered potential cathode materials in solid oxide fuel cells. In this study, the thermal evolution of thestructure, oxygen nonstoichiometry, electrical conductivity and oxygen transport properties of La2NiO4+d,Nd2NiO4+d,La3Ni2O7d,La4Ni3O10d,Pr4Ni3O10dand Nd4Ni3O10dare investigated. Phase transitionsinvolving a disruption of the cooperative tilting of the perovskite layers in the low-temperature structurethereby transforming it to a more symmetric structure are observed in several of the materials uponheating in air. Pr4Ni3O10dand Nd4Ni3O10dshow no phase transition from room temperature up to1000C. High density ceramics (>96%) are obtained after sintering at 1300C and (forn¼2 andn¼3members) post-sintering annealing at reduced temperatures. Data for the electrical conductivitymeasurements on these specimens indicate itinerant behaviour of the charge carriers in the RPnickelates. The increase in p-type conductivity with the ordernof the RP phase is interpreted as arisingfrom the concomitant increase in the formal valence of Ni. The observations can be interpreted in termsof a simple energy band scheme, showing that electron holes are formed in thesx2y2[band uponincreasing the oxidation state of Ni. Electrical conductivity relaxation measurements reveal remarkablesimilarities between the surface exchange coefficients (kchem) of the different RP phases despite thedifferences in the order parameternand the nature of the lanthanide ion. Calculation of the oxygen self-diffusion coefficients (Ds) from the experimental values of the chemical diffusion coefficients (Dchem),using the corresponding data of oxygen non-stoichiometry from thermogravimetry measurements,shows that these are strongly determined by the order parametern. The value ofDsdecreases almostone order of magnitude on going from then¼1 members La2NiO4+dand Nd2NiO4+dto then¼2member La3Ni2O7d, and again one order of magnitude on going to then¼3 members La4Ni3O10d,Pr4Ni3O10dand Nd4Ni3O10d. The results confirm that oxygen-ion transport in the investigated RPnickelates predominantly occursviaan interstitialcy mechanism within the rock-salt layer of the structures.