MS27-04 - Sub-Mesoscale Oxygen Ordering in Non-Stoichiometric Oxygen Ion Conductor Pr2NiO4+d

Paulus Werner (University of Montpellier, Institut Charles Gerhardt UMR5253, France)

Non-stoichiometric oxides can undergo important variations in the oxygen stoichiometry, enabling to tune physical and chemical properties. Combining neutron diffraction, inelastic neutron scattering and ab initio lattice dynamics calculations, we have recently evidenced the importance of lattice dynamics, i.e. soft phonon modes, triggering low temperature oxygen mobility in Brownmillerite type (Ca/Sr)Fe/CoO2.5 and Ruddlesden Popper type oxides, e.g. (Pr/Nd)2NiO4+d. [1-4]. This new concept has technological relevance for the optimization of oxygen membranes and electrolytes in SOFCs. 

We report here on single-crystal synchrotron diffraction experiments on non-stoichiometric Pr2NiO4+d, uncovering unprecedented oxygen ordering up to the sub meso-scale. Complex oxygen ordering is established during a topotactic solid-state reaction already proceeding at ambient temperature, following small oxygen release. The resulting 3D-incommensurate modulated structure is described in terms of modulation vectors and related twin domain structures. Melting of the oxygen ordering around 365°C strongly amplifies the oxygen mobility, evidenced by 18O/16O oxygen isotope exchange, and associated with phonon softening manifested in a pronounced increase of thermal diffuse scattering. Our results thus strengthen the idea of a phonon assisted oxygen diffusion mechanism, which can be more generally applied to understand cooperative diffusion mechanisms.

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[2] Perrichon, A., et al., J. Phys. Chem. C, 2015. 119(3): p. 1557-1564. 

[3] M. Ceretti et al. J. Mater. Chem. A, 3, 42 (2015) p21140-48. 

[4] M. Ceretti et al., InorganicChemistry, 2018, 57 (8), pp.4657-4666.