Neutron diffraction and DFT studies of oxygen defect and transport in higher-order Ruddlesden-Popper phase materials

A series of higher-order Ruddlesden-Popper phase materials - La₃PrNi₃O_10- , La₂Pr₂Ni₃O_10- and LaPr₃Ni₃O_10- - were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure and propose possible pathways for oxygen transport in these materials. Further complimentary DFT calculations of the materials were performed to support the experimental analysis. All of the phases were hypostoichiometric and it was observed that the majority of the oxygen vacancies were confined to the perovskite layers, with a preference for equatorial oxygen sites. A particular preference for vacancies in O(1) and O(5) sites at high temperatures was observed from neutron diffraction measurements which were further complimented by DFT calculations wherein the vacancy formation energy was found to be lowest at the O(1) site. Also, a preference for a curved oxygen transport pathway around the NiO₆ octahedra was observed which agrees with the published literature for Ruddlesden-Popper phase materials. Lattice parameters for all three compositions showed a linear increase with increasing temperature, but the increase was greatest in the c parameter while the b parameter showed only a slight increase when compared to the a parameter. The thermal expansion coefficient was calculated for all compositions and was found to be in the range 13.0-13.4 × 10^-6 °C^-1, which is compatible with the commonly used electrolyte materials for solid oxide fuel cells.