Abstract
The development of devices for energy conversion, such as solid state fuel cells, depends on the availability of materials showing high oxygen ion conduction together with low operating temperatures. Moreover, adequate structural and thermochemical stability of the pure ionic conductor membrane and the mixed ionic electronic electrodes as well as their matching at the interface are essential for the durability of the device. In this regard, the Nd2NiO4+d system proved to be a good candidate as a stable electrode for intermediate temperature solid state fuel cells.More intriguing is the fact that Nd2NiO4+d, like a few other nonstoichiometric oxides derived from the perovskite framework like the Brownmillerite-type Sr(Fe,Co)O2.5 and K2NiF4-type RE2MO4+d (RE = La, Pr and M = Ni, Cu, Co), shows oxygen ionic mobility in an electrochemical reaction at room temperature. This surprising behavior raises questions about the real microscopic transport mechanisms in these classes of materials when the temperature is as low as T=300K.A « phonon assisted diffusion » mechanism, based on the presence of a low-lying phonon modes, has been developed in Brownmillerites to describe on an atomic scale how oxygen ion diffusion can be triggered in solid oxides down to ambient temperature. Concerning the RE2MO4+d systems, oxygen conductors La2CuO4.07, Pr2NiO4.25, and Nd2NiO4.25 have been reported to show a dynamical delocalization of apical oxygen atoms of the MO6 octahedra on a circle of at least 1Å diameter. This structural instability, activated by the presence of excess oxygen, implies important shifts of apical oxygen atoms closer to vacant interstitial sites, and is thus believed to play a major role on the non-classical oxygen mobility at ambient temperature.Through this work, we have investigated, in the Nd2NiO4+d phases, the correlations between structural instabilies induced by the oxygen hyper-stoichiometry and their subsequent effects on the lattice dynamics and role in promoting oxygen diffusion in the moderate-temperature regime. We have, in particular, developed innovative approaches to investigate lattice dynamics of highly-correlated and disordered systems. Results from single-crystal diffraction, inelastic neutron scattering and first-principle simulations have evidenced that indeed the non-classical oxygen diffusion at ambient temperature can be depicted as an interplay of specific lattice-activated and single-particle motions, both directly correlated to the oxygen hyperstoichiometry. The subsequent mechanism, closely related to the “phonon assisted diffusion mechanism”, provides a comprehensive framework to describe on an atomic scale the nonclassical oxygen diffusion in non-stoichiometric oxides.