The Use of the &ldquo;+<i>U</i>&rdquo; Correction in Describing Defect States at Metal Oxide Surfaces: Oxygen Vacancies on CeO₂ and TiO₂, and Li-doping of MgO

Oxygen diffusion and vacancy migration thermally-activated govern high-temperature magnetism in ceria

Several experimental works currently demonstrate that metallic nano-oxides and carbon nanomaterials expected to be diamagnets, in fact, behave as ferromagnets at room temperature. More than scientifically intriguing, this unconventional and unexpected ferromagnetism pave the way for innovation products and novel nanotechnological applications, gathering the magnetism to interesting functionalities of these nanomaterials. Here, we investigate the non-conventional ferromagnetism observed at high temperatures in nanocrystalline cerium dioxide (CeO₂or nanoceria) thin films that are optically transparent to visible light. Nanoceria exhibits several concrete applications in catalytic processes, photovoltaic cells, solid-state fuel cells, among others, which are mostly due to natural presence of oxygen vacancies and easy migration of the oxygen through the structure. The ferromagnetism in non-stoichiometric nanocrystaline ceria can be consistently described by ab initio electronic structure calculations, which support that oxygen vacancies cause the formation of magnetic moments and can provide a robust interconnectivity within magnetic polarons theoretical framework. Additionally, we present a conceptual model to account the oxygen transport to the non-conventional ferromagnetism at temperatures well above room temperature. The approach is complementary to the thermally-activated effective transfers of charge and spin around oxygen vacancy centers.

Defects in orthorhombic LaMnO3 - ionic versus electronic compensation

The ionic and electronic conductivity of orthorhombic LaMnO3 can be modified by introducing lower valence dopants at both the La and Mn sites. Alkaline earth doped perovskites, such as LaMnO3, have a variety of applications in catalysis, for nitrogen storage and reduction, and oxidation of volatile organic compounds, and as the oxygen electrode in solid oxide fuel cells. Here, we investigate doping with the divalent alkaline earth metals Mg, Ca, Sr and Ba, and the charge compensation mechanism. The energies of formation of isolated defects and clustered pairs were investigated at both La and Mn sites to establish the most probable site at which they will be introduced. The charge compensation mechanism for the introduction of alkaline earth dopants was examined by considering both ionic (formation of an oxygen vacancy for every two alkaline earth dopants introduced) and electronic compensation (a hole localised at the Mn site for each dopant introduced). Larger cations (Ca, Sr and Ba) were found to have lower defect formation energies when introduced at the La site, while the smaller Mg defect had lower formation energies when introduced to the Mn site. For all defects introduced, electronic compensation for the defect was found to be more energetically favourable, which will result in improved electronic conductivity of the material.

The electronic properties of Au clusters on CeO2 (110) surface with and without O-defects

We use density functional theory with Hubbard corrections (DFT+U) to understand the local electronic properties of Au adatom and Au₂ dimer adsorption on the CeO₂ (110) surface with and without O-defects.

The adsorption of Cu on the CeO2(110) surface

A detailed density functional theory (DFT) study coupled with extended X-ray absorption fine structure (EXAFS) experiments on the geometrical and electronic properties of copper species on CeO₂ surface demonstrating the effects of oxidation state and solvent environment.

Occupation matrix control of d- and f-electron localisations using DFT + U

The use of a density functional theory methodology with on-site corrections (DFT + U) has been repeatedly shown to give an improved description of localised d and f states over those predicted with a standard DFT approach. However, the localisation of electrons also carries with it the problem of metastability, due to the possible occupation of different orbitals and different locations. This study details the use of an occupation matrix control methodology for simulating localised d and f states with a plane-wave DFT + U approach which allows the user to control both the site and orbital localisation. This approach is tested for orbital occupation using octahedral and tetrahedral Ti(iii) and Ce(iii) carbonyl clusters and for orbital and site location using the periodic systems anatase-TiO2 and CeO2. The periodic cells are tested by the addition of an electron and through the formation of a neutral oxygen vacancy (leaving two electrons to localise). These test systems allow the successful study of orbital degeneracies, the presence of metastable states and the importance of controlling the site of localisation within the cell, and it highlights the use an occupation matrix control methodology can have in electronic structure calculations.