Long-range oxygen ordering linked to topotactic oxygen release in Pr2NiO4+δ fuel cell cathode material

Solid oxide ion conductors are technologically important for oxygen membranes, sensors and solid oxide fuel cells (SOFC). However, oxygen diffusion is a thermally activated process, and materials operating at ambient temperature are rare, as the related diffusion mechanisms are poorly understood. Herein, we report a hidden spontaneous oxygen release reaction that interconverts two stoichiometric phases of the SOFC material Pr2NiO4+δ with unprecedented structural complexity at ambient temperature. A slight change in the oxygen stoichiometry from δ = 0.25 to δ = 0.225 involves a transition between two competing modulated superstructures, showing long-range translational periodicities up to 94 Å. Our findings demonstrate correlated oxygen diffusion within the bulk phase at room temperature, which is accompanied by the formation of long-range modulated superstructures up to the lower mesoscale. Large-scale ordered structures found here for Pr2NiO4+δ are rather indicative of non-local interactions, and are interpreted to be mediated via structural deformations. This unexpected behavior is discussed in terms of an unconventional understanding of low-T oxygen diffusion mechanisms and its potential to conceive and optimize oxygen ion conductors, which are an important class of compounds relevant to technological applications.Mécanismes assistés pour la conduction par ion oxygène dans des oxydes non stœchiométriquesStructural induced Electronic Complexity controlled by low temperature Topotactic Reactio