Theoretical study of the stabilization of cubic-phase ZrO2 by impurities

Structure and Chemical Reactivity of Y-Stabilized ZrO2 Surfaces: Importance for the Water-Gas Shift Reaction

The surface structure and chemical properties of Y-stabilized zirconia (YSZ) have been subjects of intense debate over the past three decades. However, a thorough understanding of chemical processes occurring at YSZ powders faces significant challenges due to the absence of reliable reference data acquired for well-controlled model systems. Here, we present results from polarization-resolved infrared reflection absorption spectroscopy (IRRAS) obtained for differently oriented, Y-doped ZrO2 single-crystal surfaces after exposure to CO and D2O. The IRRAS data reveal that the polar YSZ(100) surface undergoes reconstruction, characterized by an unusual, red-shifted CO band at 2132 cm-1. Density functional theory calculations allowed to relate this unexpected observation to under-coordinated Zr4+ cations in the vicinity of doping-induced O vacancies. This reconstruction leads to a strongly increased chemical reactivity and water spontaneously dissociates on YSZ(100). The latter, which is an important requirement for catalysing the water-gas-shift (WGS) reaction, is absent for YSZ(111), where only associative adsorption was observed. Together with a novel analysis Scheme these reference data allowed for an operando characterisation of YSZ powders using DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). These findings facilitate rational design and tuning of YSZ-based powder materials for catalytic applications, in particular CO oxidation and the WGS reaction.

Influence of Th, Zr, and Ti Dopants on Solution Property of Xe in Uranium Dioxide with Defects: A DFT + U Study

To ensure the safety and efficient operation of nuclear reactors, it is imperative to understand the effects of various dopants (Ti, Th, and Zr) on the solubility of the fission product Xe in UO2. In this study, Hubbard corrected density functional theory (DFT + U) and occupation matrix control were used to investigate the bulk and defect properties of UO2. The results show that the UO2-Ti system is more favorable for Xe dissolution in vacancies, whereas the UO2-Th system has little effect on the dissolution of Xe atoms. Th, Zr, and Ti inhibit the aggregation of Xe clusters, and Ti is the least favorable for the nucleation and growth of Xe clusters.

Copper–Zirconia Catalysts: Powerful Multifunctional Catalytic Tools to Approach Sustainable Processes

Copper–zirconia catalysts find many applications in different reactions owing to their unique surface properties and relatively easy manufacture. The so-called methanol economy, which includes the CO2 and CO valorization and the hydrogen production, and the emerging (bio)alcohol upgrading via dehydrogenative coupling reaction, are two critical fields for a truly sustainable development in which copper–zirconia has a relevant role. In this review, we provide a systematic view on the factors most impacting the catalytic activity and try to clarify some of the discrepancies that can be found in the literature. We will show that contrarily to the large number of studies focusing on the zirconia crystallographic phase, in the last years, it has turned out that the degree of surface hydroxylation and the copper–zirconia interphase are in fact the two mostly determining factors to be controlled to achieve high catalytic performances.

Electronic Band Structure Variations in the Ceria Doped Zirconia: A First Principles Study

Using first principle calculations, the effect of Ce with different doping concentrations in the network of Zirconium dioxide (ZrO₂) is studied. The ZrO₂ cell volume linearly increases with the increasing Ce doping concentration. The intrinsic band gap of ZrO₂ of 5.70 eV reduces to 4.67 eV with the 2.08% Ce doping. In 4.16% cerium doped ZrO₂, the valence band maximum and conduction band minimum come closer to each other, about 1.1 eV, compared to ZrO₂. The maximum band gap reduction of ZrO₂ is observed at 6.25% Ce doping concentration, having the value of 4.38 eV. No considerable shift in the band structure is found with further increase in the doping level. The photo-response of the ZrO₂ is modulated with Ce insertion, and two distinct modifications are observed in the absorption coefficient: an imaginary part of the dielectric function and conductivity. A 2.08% Ce-doped ZrO₂ modeled system reduces the intensities of peaks in the optical spectra while keeping the peaks of intrinsic ZrO₂. However, the intrinsic peaks related to ZrO₂ completely vanish in 4.16%, 6.25%, 8.33%, and 12.5% Ce doped ZrO₂, and a new absorption hump is created.

Revealing the properties of the cubic ZrO2 (111) surface by periodic DFT calculations: reducibility and stabilization through doping with aliovalent Y2O3

The reducibility of the clean cubic ZrO₂ (111) surface, as well as its stabilization through doping, have been investigated by hybrid DFT calculations within a periodic approach and localized basis sets.

Tuning the charge state of Ag and Au atoms and clusters deposited on oxide surfaces by doping: a DFT study of the adsorption properties of nitrogen- and niobium-doped TiO2 and ZrO2

Defects (O vacancies) and dopants (nitrogen and niobium impurities) in titania and zirconia affect the properties of adsorbed Ag and Au clusters.

Chemical evolution via beta decay: a case study in strontium-90

Using (90)Sr as a representative isotope, we present a framework for understanding beta decay within the solid state. We quantify three key physical and chemical principles, namely momentum-induced recoil during the decay event, defect creation due to physical displacement, and chemical evolution over time. A fourth effect, that of electronic excitation, is also discussed, but this is difficult to quantify and is strongly material dependent. The analysis is presented for the specific cases of SrTiO(3) and SrH(2). By comparing the recoil energy with available threshold displacement data we show that in many beta-decay situations defects such as Frenkel pairs will not be created during decay as the energy transfer is too low. This observation leads to the concept of chemical evolution over time, which we quantify using density functional theory. Using a combination of Bader analysis, phonon calculations and cohesive energy calculations, we show that beta decay leads to counter-intuitive behavior that has implications for nuclear waste storage and novel materials design.

Investigating the electronic structure of fluorite-structured oxide compounds: comparison of experimental EELS with first principles calculations

Energy loss spectra from fluorite-structured ZrO(2), CeO(2), and UO(2) compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.

Local structure and ionic conductivity in the Zr(2)Y(2)O(7)-Y(3)NbO(7) system

The Zr(0.5-0.5x)Y(0.5+0.25x)Nb(0.25x)O(1.75) solid solution possesses an anion-deficient fluorite structure across the entire 0≤x≤1 range. The relationship between the disorder within the crystalline lattice and the preferred anion diffusion mechanism has been studied as a function of x, using impedance spectroscopy measurements of the ionic conductivity (σ), powder neutron diffraction studies, including analysis of the 'total' scattering to probe the nature of the short-range correlations between ions using reverse Monte Carlo (RMC) modelling, and molecular dynamics (MD) simulations using potentials derived with a strong ab initio basis. The highest total ionic conductivity (σ = 2.66 × 10(-2) Ω(-1) cm(-1) at 1473 K) is measured for the Zr(2)Y(2)O(7) (x = 0) end member, with a decrease in σ with increasing x, whilst the neutron diffraction studies show an increase in lattice disorder with x. This apparent contradiction can be understood by considering the local structural distortions around the various cation species, as determined from the RMC modelling and MD simulations. The addition of Nb(5+) and its stronger Coulomb interaction generates a more disordered local structure and enhances the mobility of some anions. However, the influence of these pentavalent cations is outweighed by the effect of the additional Y(3+) cations introduced as x increases, which effectively trap many anions and reduce the overall concentration of the mobile O(2-) species.

Free-energy calculations for the cubic ZrO2 crystal as an example of a system with a soft mode

We calculate the free energy for a crystalline ZrO(2) with a soft mode by the first-principles method, using the double-well energy-displacement relation. The soft-mode branch is considered as an ensemble of independent anharmonic oscillators of the parabola-plus-Gaussian or of the 2-4 polynomial forms. The anharmonic contributions are included to reproduce the cubic-to-tetragonal phase transition, however, it appears that the cubic phase does not become the most stable within the framework of the independent oscillators approach.

Lattice vibrations in cubic, tetragonal, and monoclinic phases of ZrO2

First principles calculations of the phonon dispersion relations and the phonon density of states for three zero-pressure zirconia phases are presented. The phonon dispersion relations of the tetragonal and monoclinic phases do not exhibit the imaginary frequencies, contrary to the cubic phase for which the imaginary soft mode is seen. For tetragonal and monoclinic phases the free energies versus temperature are calculated in harmonic approximation. They cross at 1560 K indicating the phase transition.

"Non-VSEPR" Structures and Bonding in d(0) Systems

Under certain circumstances, metal complexes with a formal d(0) electronic configuration may exhibit structures that violate the traditional structure models, such as the VSEPR concept or simple ionic pictures. Some examples of such behavior, such as the bent gas-phase structures of some alkaline earth dihalides, or the trigonal prismatic coordination of some early transition metal chalcogenides or pnictides, have been known for a long time. However, the number of molecular examples for "non-VSEPR" structures has increased dramatically during the past decade, in particular in the realm of organometallic chemistry. At the same time, various theoretical models have been discussed, sometimes controversially, to explain the observed, unusual structures. Many d(0) systems are important in homogeneous and heterogeneous catalysis, biocatalysis (e.g. molybdenum or tungsten enzymes), or materials science (e.g. ferroelectric perovskites or zirconia). Moreover, their electronic structure without formally nonbonding d orbitals makes them unique starting points for a general understanding of structure, bonding, and reactivity of transition metal compounds. Here we attempt to provide a comprehensive view, both of the types of deviations of d(0) and related complexes from regular coordination arrangements, and of the theoretical framework that allows their rationalization. Many computational and experimental examples are provided, with an emphasis on homoleptic mononuclear complexes. Then the factors that control the structures are discussed in detail. They are a) metal d orbital participation in sigma bonding, b) polarization of the outermost core shells, c) ligand repulsion, and d) pi bonding. Suggestions are made as to which of the factors are the dominant ones in certain situations. In heteroleptic complexes, the competition of sigma and pi bonding of the various ligands controls the structures in a complicated fashion. Some guidelines are provided that should help to better understand the interrelations. Bent's rule is of only very limited use in these types of systems, because of the paramount influence of pi bonding. Finally, computed and measured structures of multinuclear complexes are discussed, including possible consequences for the properties of bulk solids.