Lattice Dynamics Modified by Excess Oxygen in Nd2NiO4+δ: Triggering Low-Temperature Oxygen Diffusion
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Abstract
Nonstoichiometric Nd2NiO4+δ shows oxygen ion mobility already at room temperature and as such is a promising material in the field of solid state ionic conductors. The present work aims to understand the impact of oxygen overstoichiometry on the lattice dynamics of the Nd2NiO4 framework and to correlate dynamic instabilities to the anionic mobility at room temperature. We performed neutron time-of-flight spectroscopy experiments coupled with DFT-based molecular dynamics calculations on the phases Nd2NiO4.0, Nd2NiO4.10, and Nd2NiO4.25. Specific signatures in the phonon density of states gave evidence that excess oxygen on interstitial lattice sites activates large displacements of the apical oxygen atoms along the [110] direction, thus favoring their diffusion toward interstitial sites within the rock salt-type layer already at ambient temperature. We implemented a position recurrence method to analyze apical oxygen displacements during the 40 ps time scale of molecular dynamics simulations. The results highlighted that dynamical delocalization of apical oxygen atoms along [110] is necessary to observe diffusion at room temperature, and thus ionic mobility at room temperature is activated by lattice dynamics.
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