Application of Density Functional Theory to the Modeling of the Mixed Ionic and Electronic Conductor La₂NiO_4+δ: Lattice Relaxation, Oxygen Mobility, and Energetics of Frenkel Defects

The uptake of oxygen ions into the La 2 NiO 4 host matrix was investigated by electronic structure calculations within density functional theory (DFT). For La 2 NiO 4.125, considered in a tetragonal simulation supercell, a tilting phenomenon occurs, alternating one NiO 2 plane on two along the [100] and [010] directions of the parent I 4/ mmm lattice. Beyond structural considerations, DFT calculations are used in conjunction with a topological analysis of the electron density to determine, at the atomic scale, the intrinsic features governing ionic conductivity for a direct interstitial mechanism within the (001) plane. Lattice relaxation and oxygen polarizability seem to be the key factors governing direct diffusion mechanisms in this compound. Charge-transfer phenomena were ruled out as a relevant parameter for oxygen mobility. The interstitial oxygen is identified as a formal O 2 - species both at its usual crystallographic site and at the highly constrained saddle point state. Oxygen vacancies at the equatorial sites are found to be energetically more favorable than at the apical sites for the formation of Frenkel-type defects.