43 research outputs found
Quantum Mechanical Calculations Of Elastic Properties Of Doped Tetragonal Yttria-Stabilized Zirconium Dioxide
We report ïŹrst principles calculations of the electronic and elastic properties of yttriastabilized tetragonal zirconium dioxide doped with metal oxides like: GeO2, TiO2, SiO2,MgO and Al2O3. It is shown that addition of such dopants aïŹects selected elastic propertiesof ZrO2, which is driven by the attraction of electron density by dopant atom and creationof stronger dopantâoxygen bonds. This eïŹect contributes to the increase of superplasticityof doped material
When Langmuir is too simple: H-2 dissociation on Pd(111) at high coverage
Recent experiments of H2 adsorption on Pd(111) [T. Mitsui et al., Nature (London) 422, 705 (2003)] have questioned the classical Langmuir picture of second order adsorption kinetics at high surface coverage requiring pairs of empty sites for the dissociative chemisorption. Experiments find that at least three empty sites are needed. Through density functional theory, we find that H2 dissociation is favored on ensembles of sites that involve a Pd atom with no direct interaction with adsorbed hydrogen. Such active sites are formed by aggregation of at least 3 H-free sites revealing the complex structure of the "active sites.
Performance of DFT+U Approaches in the Study of Catalytic Materials
Catalytic materials are complex systems that contain different units, typically a carrier and an active phase. For many reactions, redox-active materials constitute the active phase. Active catalytic phases based on metal oxides include, but are not limited to, silver, copper, vanadium, molybdenum, iron, cobalt, and titanium oxides, and most notably ceria.(1, 2) This results in variable occupations of the d or f states of the cations. Therefore, the proper energy alignment of these states is mandatory for the adequate description of the chemistry responsible for the catalytic processes. In particular, all the challenges related to energy-harvesting and storage technologies are intrinsically linked to the small energy difference between the different spin configurations, the easy transfer between them, and the correct alignment of the energy states
Binding in alkali and alkaline-earth tetrahydroborates: Special position of magnesium tetrahydroborate
Compounds of light elements and hydrogen are currently extensively studied due to their potential applica- tion in the field of hydrogen or energy storage. A number of new interesting tetrahydroborates that are especially promising due to their very high gravimetric hydrogen content were recently reported. However, the determination and understanding of their complex crystalline structures has created considerable debate. Metal tetrahydroborates, in general, form a large variety of structures ranging from simple for NaBH4 to very complex for Mg BH4 2. Despite the extensive discussion in the literature no clear explanation has been offered for this variety so far. In this paper we analyze the structural and electronic properties of a broad range of metal tetrahydroborates and reveal the factors that determine their structure: ionic bonding, the orientation of the BH4 groups, and the coordination number of the metal cation. We show, in a simple way, that the charge transfer in the metal tetrahydroborates rationally explains the structural diversity of these compounds. Being ionic sys- tems, the metal tetrahydroborates fall into the classification of Linus Pauling. By using the ionic radius for the BH4 group as determined in this paper, this allows for structural predictions for new and mixed compounds
Understanding and Tuning the Intrinsic Hydrophobicity of Rare- Earth Oxides: A DFT+U Study
Rare-earth oxides (REOs) possess a remarkable
intrinsic hydrophobicity, making them candidates for a myriad
of applications. Although the superhydrophobicity of REOs has
been explored experimentally, the atomistic details of the
structure at the oxideâwater interface are still not well
understood. In this work, we report a density functional theory
study of the interaction between water and CeO2, Nd2O3, and
α-Al2O3 to explain their different wettability. The wetting of the
metal oxide surface is controlled by geometric and electronic
factors. While the electronic term is related to the acidâbase
properties of the surface layer, the geometric factor depends on
the matching between adsorption sites and oxygen atoms from
the hexagonal water network. For all the metal oxides
considered here, water dissociation is confined to the first
oxide-water layer. Hydroxyl groups on α-Al2O3 are responsible for the strong oxideâwater interaction, and thus, both Al- and hydroxyl-terminated wet. On CeO2, the intrinsic hydrophobicity of the clean surface disappears when lattice hydroxyl groups (created by the reaction of water with oxygen vacancies) are present as they dominate the interaction and drive wetting. Therefore, hydroxyls may convert a intrinsic nonwetting surface into a wetting one. Finally, we also report that surface modifications, like cation substitution, do not change the acidâbase character of the surface, and thus they show the same nonwetting properties as native CeO2 or Nd2O3
NidoâHydroborateâBased Electrolytes for AllâSolidâState Lithium Batteries
Hydroborateâbased solid electrolytes have recently been successfully employed in high voltage, room temperature allâsolidâstate sodium batteries. The transfer to analogous lithium systems has failed up to now due to the lower conductivity of the corresponding lithium compounds and their high cost. Here LiB11H14 nidoâhydroborate as a costâeffective building block and its highâpurity synthesis is introduced. The crystal structures of anhydrous LiB11H14 as well as of LiB11H14âbased mixedâanion solid electrolytes are solved and high ionic conductivities of 1.1 Ă 10â4 S cmâ1 for Li2(B11H14)(CB11H12) and 1.1 Ă 10â3 S cmâ1 for Li3(B11H14)(CB9H10)2 are obtained, respectively. LiB11H14 exhibits an oxidative stability limit of 2.6 V versus Li+/Li and the proposed decomposition products are discussed based on density functional theory calculations. Strategies are discussed to improve the stability of these compounds by modifying the chemical structure of the nidoâhydroborate cage. Galvanostatic cycling in symmetric cells with two lithium metal electrodes shows a small overpotential increase from 22.5 to 30 mV after 620 h (up to 0.5 mAh cmâ2), demonstrating that the electrolyte is compatible with metallic anodes. Finally, the Li2(B11H14)(CB11H12) electrolyte is employed in a proofâofâconcept half cell with a TiS2 cathode with a capacity retention of 82% after 150 cycles at C/5