A systematic evaluation of the role of lanthanide elements in functional complex oxides; Implications for energy conversion devices

Lanthanide containing complex oxides, especially the ABO₃ perovskite and A_(n+1)B_nO_(3n+1) Ruddlesden-Popper series, attract much interest as promising catalytic materials in many renewable energy applications such as electro-chemical energy conversion and hydrogen production. Recent experimental and theoretical studies on some members of these materials, e.g. La₂NiO₄, revealed that the La-O terminated surfaces are catalytically active under operational conditions. These findings suggested that the conventional understanding of such oxides being fully ionized, and composed of catalytically inert La³⁺ ions needs to be revised. In this study, generalized gradient approximation and hybrid density functional theory methods were used to study and compare the electronic structures of La and Sr in related oxides. Density functional theory approaches based on both Gaussian and plane wave basis sets were employed to ensure robustness of this study. Consistent results were obtained across different ab initio methods and approaches used. Density of states plots and charge analysis results showed that La exhibits a partially occupied d-orbital and an atomic charge of +2 instead of its nominal valence number (+3) in the oxides, while Sr does not show similar characteristics. Electron density maps obtained from synchrotron X-ray diffraction experiments confirmed the simulation findings as well. The presence of the available d-orbital electron on La and associated partial covalency were postulated as being responsible for the catalytic behaviour observed in experiments. In addition, Pr and Ba electronic structures in related oxides were also calculated. A similar trend to the La and Sr charges was observed. Based on these findings, the traditional concept of atomic "ionicity" was briefly reviewed and adapted as a catalysis descriptor for possible performance evaluation.