Electronic and magnetic properties of Co and Ni impurities in Cu wires: first-principles investigation of local moment formation in one dimension

"The local moment formation in one-dimensional (1D) systems is investigated in the framework of a generalized gradient approximation to density-functional theory (DFT). The electronic and magnetic properties of Co and Ni impurities in finite Cu wires are determined as a function of experimentally relevant parameters such as wire length, impurity-host distance, impurity position within the wire, and total spin polarization S-z. Results are given for the interatomic equilibrium distances, relative stability of different total spin configurations, local magnetic moments in both Wigner-Seitz and Bader cells, electronic density of states at the impurity, and induced magnetic moments in the 1D metal including their coupling with the impurity. The calculations show that the optimal total spin polarization is one above the minimal value. In fact, for chains having an even number of Cu atoms, the ground-state total spin is S-z = 1 for Ni-doped wires and S-z = 3/2 for Co-doped wires. Both Co and Ni impurities preserve their magnetic degree of freedom and develop large local magnetic moments in all low-lying spin configurations (S-z <= 5/2). These almost completely saturated impurity moments are largely dominated by the d-electron contributions. In the ground-state the magnetic coupling between the impurity and the induced moments at the host atoms is ferromagneticlike. Thus, the local exchange energy dominates over hybridization and spin-fluctuation effects, at least in the framework of the present approximation to DFT. The local density of electronic states (LDOS) at the impurity is found to have essentially d character in the whole valence-band range. Large exchange splittings consistent with saturated d moments are observed, which imply a full minority-spin polarization of the LDOS at the Fermi energy epsilon(F). A remarkable correlation is revealed between the rotational symmetry and the degree of delocalization of the impurity states close to epsilon(F). Trends as a function of the local atomic environment are discussed.

Activation of surface oxygen sites on an iridium-based model catalyst for the oxygen evolution reaction

International audienceThe oxygen evolution reaction (OER) is of prime importance in multiple energy storage devices, however, deeper mechanistic understanding is required in order to design enhanced electrocatalysts for the reaction. Current understanding of the OER mechanism based on oxygen adsorption on a metallic surface site fails to fully explain the activity of iridium and ruthenium oxide surfaces, and the drastic surface reconstruction observed for the most active OER catalysts. Here we demonstrate, using La 2 LiIrO 6 as a model catalyst, that the exceptionally high activity found for Ir-based catalysts arises from the formation of active surface oxygen atoms that act as electrophilic centres for water to react. Moreover, we observe with the help of transmission electron microscopy drastic surface reconstruction and iridium migration from the bulk to the surface. Therefore, we establish a correlation between surface activity and surface stability for OER catalysts which is rooted in the formation of surface reactive oxygen

Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries

International audienceLithium-ion (Li-ion) batteries that rely on cationic redox reactions are the primary energy source for portable electronics. One pathway toward greater energy density is through the use of Li-rich layered oxides. The capacity of this class of materials (>270 milliampere hours per gram) has been shown to be nested in anionic redox reactions, which are thought to form peroxo-like species. However, the oxygen-oxygen (O-O) bonding pattern has not been observed in previous studies, nor has there been a satisfactory explanation for the irreversible changes that occur during first delithiation. By using Li2IrO3 as a model compound, we visualize the O-O dimers via transmission electron microscopy and neutron diffraction. Our findings establish the fundamental relation between the anionic redox process and the evolution of the O-O bonding in layered oxides