Structure, electrical conductivity and oxygen transport properties of Ruddlesden–Popper phases Lnn+1NinO3n+1 (Ln = La, Pr and Nd; n = 1, 2 and 3)

Abstract

Layered Ruddlesden–Popper (RP) lanthanide nickelates, Ln_n+1Ni_nO_3n+1 (Ln = La, Pr and Nd; n = 1, 2 and 3), are considered potential cathode materials in solid oxide fuel cells. In this study, the thermal evolution of the structure, oxygen nonstoichiometry, electrical conductivity and oxygen transport properties of La₂NiO_4+δ, Nd₂NiO_4+δ, La₃Ni₂O_7−δ, La₄Ni₃O_10−δ, Pr₄Ni₃O_10−δ and Nd₄Ni₃O_10−δ are investigated. Phase transitions involving a disruption of the cooperative tilting of the perovskite layers in the low-temperature structure thereby transforming it to a more symmetric structure are observed in several of the materials upon heating in air. Pr₄Ni₃O_10−δ and Nd₄Ni₃O_10−δ show no phase transition from room temperature up to 1000 °C. High density ceramics (>96%) are obtained after sintering at 1300 °C and (for n = 2 and n = 3 members) post-sintering annealing at reduced temperatures. Data for the electrical conductivity measurements on these specimens indicate itinerant behaviour of the charge carriers in the RP nickelates. The increase in p-type conductivity with the order n of the RP phase is interpreted as arising from the concomitant increase in the formal valence of Ni. The observations can be interpreted in terms of a simple energy band scheme, showing that electron holes are formed in the σ_x²−y²↑ band upon increasing the oxidation state of Ni. Electrical conductivity relaxation measurements reveal remarkable similarities between the surface exchange coefficients (k_chem) of the different RP phases despite the differences in the order parameter n and the nature of the lanthanide ion. Calculation of the oxygen self-diffusion coefficients (D_s) from the experimental values of the chemical diffusion coefficients (D_chem), using the corresponding data of oxygen non-stoichiometry from thermogravimetry measurements, shows that these are strongly determined by the order parameter n. The value of D_s decreases almost one order of magnitude on going from the n = 1 members La₂NiO_4+δ and Nd₂NiO_4+δ to the n = 2 member La₃Ni₂O_7−δ, and again one order of magnitude on going to the n = 3 members La₄Ni₃O_10−δ, Pr₄Ni₃O_10−δ and Nd₄Ni₃O_10−δ. The results confirm that oxygen-ion transport in the investigated RP nickelates predominantly occurs via an interstitialcy mechanism within the rock-salt layer of the structures.