A systematic ab initio study of vacancy formation energy, diffusivity, and ionic conductivity of Ln2NiO4+s (Ln=La, Nd, Pr)

This study systematically investigates the vacancy formation energy, diffusivity, and ionic conductivity of the Ln2NiO4+s (Ln = La, Nd, Pr, and s = 0.125) compound using the ab initio approach. Specifically, the impact of thermal expansion on the oxygen transport properties is considered, using a combination of quasi-harmonic approximation (QHA) and a linear regression model to study and reproduce the temperature-dependent prop-erties of Ln2NiO4+s. Overall, the predictions are in excellent agreement with previous theoretical studies in the literature. It is confirmed that the ionic transport properties of the Ln2NiO4+s are not dominated by oxygen vacancy diffusion due to the high vacancy formation energy. Additionally, the interstitialcy mechanism, which involves interstitial O2-hopping parallel to the a-b plane with the rocksalt layer, is determined to be the more favorable diffusion path. Meanwhile, the predicted energy barrier, diffusion coefficient, and ionic conductivity of Ln2NiO4+s show reasonable agreement with experimental data, with Pr2NiO4+s exhibiting the lowest activation energy barrier (Delta Eb = 0.722 eV) and the highest thermal expansion, diffusivity, and ionic conductivity. Overall, this study presents an efficient and computationally facile tool for predicting ionic transport properties in ma-terials where thermal expansion is the main driving force for temperature-dependent properties.