David Muñoz Ramo

B3LYP calculations of cerium oxides

In this paper we evaluate the performance of density functional theory with the B3LYP functional for calculations on ceria (CeO(2)) and cerium sesquioxide (Ce(2)O(3)). We demonstrate that B3LYP is able to describe CeO(2) and Ce(2)O(3) reasonably well. When compared to other functionals, B3LYP performs slightly better than the hybrid functional PBE0 for the electronic properties but slightly worse for the structural properties, although neither performs as well as LDA+U(U=6 eV) or PBE+U(U=5 eV). We also make an extensive comparison of atomic basis sets suitable for periodic calculations of these cerium oxides. Here we conclude that there is currently only one type of cerium basis set available in the literature that is able to give a reasonable description of the electronic structure of both CeO(2) and Ce(2)O(3). These basis sets are based on a 28 electron effective core potential (ECP) and 30 electrons are attributed to the valence space of cerium. Basis sets based on 46 electron ECPs fail for these materials.

Spin crossover in Fe2SiO4 liquid at high pressure

We combine spin-polarized density functional theory with first principle molecular dynamics (FPMD) to study the spin crossover in liquid Fe2SiO4, up to 300 GPa and 6000 K. In contrast to the much sharper transition seen in crystals, we find that the high- to low-spin transition occurs over a very broad pressure interval (>200 GPa) due to structural disorder in the liquid. We find excellent agreement with the experimental Hugoniot. We combine our results with previous FPMD calculations to derive the partial molar volumes of the oxide components MgO, FeO, and SiO 2 . We find that eutectic melts in the MgO-FeO-SiO 2 system are denser than coexisting solids in the bottom 600 km of Earths mantle.

Structure and spectroscopic properties of oxygen divacancy in yttrium-stabilized zirconia

We have studied the structure and spectroscopic properties of the oxygen divacancy defect in Yttrium-stabilized ZrO2 using periodic and embedded cluster methods and GGA and B3LYP density functionals. The results demonstrate that the defect spectroscopic properties depend on the particular arrangement of Y dopants near vacancies. The optical transition energies calculated for the negatively charged state of the divacancy at 2.8 eV and 3.3 eV are in agreement with experimental data. The second set of transitions between 1.9 eV and 2.7 eV corresponds to the electron transfer between vacancies. The calculated EPR g-tensor values are in agreement with other works. The results support the proposed attribution of the optical absorption peaking at 3.3 eV and related EPR spectra to Zr3+ ions in the YSZ matrix, however, they are not fully conclusive due to dependence on Y concentration.

The effect of defects and disorder on the electronic properties of ZnIr2O4

We analyze by means of ab initio calculations the role of imperfections on the electronic structure of ZnIr2O4, ranging from point defects in the spinel phase to the fully amorphous phase. We find that interstitial defects and anion vacancies in the spinel have large formation energies, in agreement with the trends observed in other spinels. In contrast, cation vacancies and antisites have lower formation energies. Among them, the zinc antisite and the zinc vacancy are the defects with the lowest formation energy. They are found to act as acceptors, and may be responsible for the spontaneous hole doping in the material. They may also induce optical transitions that would reduce the transparency of the material. Amorphization of ZnIr2O4 leads a large decrease of the band gap and appearance of localized states at the edges of the band gap region, which may act as charge traps and prevent amorphous ZnIr2O4 from being a good hole conductor.

Impact of amorphization on the electronic properties of Zn-Ir-O systems.

We analyze the geometry and electronic structure of a series of amorphous Zn-Ir-O systems using classical molecular dynamics followed by density functional theory taking into account two different charge states of Ir (+3 and  +4). The structures obtained consist of a matrix of interconnected metal-oxygen polyhedra, with Zn adopting preferentially a coordination of 4 and Ir a mixture of coordinations between 4 and 6 that depend on the charge state of Ir and its concentration. The amorphous phases display reduced band gaps compared to crystalline ZnIr2O4 and exhibit localized states near the band edges, which harm their transparency and hole mobility. Increasing amounts of Ir in the Ir(4+) phases decrease the band gap further while not altering it significantly in the Ir(3+) phases. The results are consistent with recent transmittance and resistivity measurements.