Non-Catalyzed and Pt/γ-Al2O3 Catalyzed Hydrothermal Cellulose Dissolution-Conversion: Influence of the Reaction Parameters

The dissolution of cellulose under 5 MPa of H₂ in the absence of catalyst is temperature and time dependant. The presence of Pt/γ-Al₂O₃ increases the initial rate of dissolution. The presence of H₂/Pt is essential although its exact role has not been well elucidated.     

Influence of Stoichiometry on the Electrical and Mechanical Behavior of Yttrium Oxide Ceramics

Because of their method of preparation, ceramics contain point defects, the concentration of which may vary from the center to the boundary of the grains, which may change the bulk properties of the material. The electronic structure of the oxygen-related defects has been studied by electron spectroscopy in yttria ceramics submitted to various heating conditions under controlled atmospheres. The differences in the mechanical and electrical behaviors are observed to correlate with the defect content.     

α-Al2O3 (0001) Surfaces: Atomic and Electronic Structure

New results on the     R ± 9° reconstructed α-Al₂O₃ (0001) surface, which can be obtained after heating at high temperature (1400°C) under vacuum, are presented. The atomic structure has been studied by combining low-energy electron diffractometry and grazing incidence X-ray scattering. The surface structure is found to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-packed Al (111) planes separated by a defect of hexagonal periodicity with a parameter of 26.44 Å. This model is consistent with previous surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ray absorption spectroscopy at the O K edge, electron energy loss spectroscopy, and X-ray-induced Auger electron spectroscopy. Interpretation of these experimental data in the frame of a self-consistent, tight-binding calculation leads to the conclusion that the     R ± 9° reconstructed surface is more covalent than the (1 × 1) surface. Significant changes in the; Al-O hybridizations are observed; these are likely due to a difference in the interatomic distances along the [0001] axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstructed surface.     

Influence of Oxygen Vacancies on the Electronic Structure of Yttrium Oxide

The influence of oxygen vacancies on the electronic structure of yttrium oxide was investigated both experimentally and theoretically. By means of X-ray absorption spectroscopy, at the yttrium K and L ₁₁ edges and at the oxygen K edge, information on the local densities of states of various orbital symmetries, around each type of atom, was obained. The total density of states in the valence band was studied using X-ray photoelectron spectroscopy. Experimental results were compared with those obtained on stoichiometric yttria. The evolution of the densities of states is well reproduced by self-consistent, semiempirical tight-binding calculations applied to clusters of increasing size, in which oxygen vacancies are introduced. The charge transfer between oxygen and yttrium is modified, as experimentally observed from the study of the O KLL Auger line shape. The decrease of the local atomic charge on oxygen is theoretically confirmed by the tight-binding calculations, showing a tendency toward a more covalent bond in nonstoichiometric yttria.     

Electronic structure packages: Two implementations of the projector augmented wave (PAW) formalism

The projector augmented wave (PAW) formalism developed by Blöchl [Phys. Rev. B 50 (1994) 17953] is an accurate and efficient pseudopotential-like scheme for electronic structure calculations within density functional theory and is now implemented in several electronic structure codes. Some of these codes use an implementation of the formalism developed by Kresse et al. [Phys. Rev. B 59 (1999) 1758] which differs slightly from the original Blöchl formalism and which can lead to different electronic structure results. In this paper, we analyze and illustrate the difference between the Blöchl and Kresse PAW formulations.     

Nonstoichiometry in Pure and Zr-Doped Yttria Ceramics: An EXAFS and XPS Study

Nonstoichiometry is exhibited by many ceramics at very high temperatures. The high-temperature treatment of yttria, in particular, can induce a large concentration of point defects that changes the electrical and mechanical behavior of the solid. In the present study, the electronic and geometric structures of nonstoichiometric pure and Zr-doped yttria are examined by X-ray photoelectron spectroscopy and X-ray absorption. The results show that the doping atom influences the level of stoichiometry, as well as the induced disorder and the change in the partial ionic character of the Y-O bond.     

Generation of Projector Augmented-Wave atomic data: A 71 element validated table in the XML format

In the Projector Augmented Wave (PAW) method developed by Blöchl (1994), a PAW data file is needed for each element, taking the role of the pseudopotential file used with the norm-conserving or ultrasoft formalisms. In this paper, we review methods for generating PAW data files and for evaluating their accuracy, transferability, and numerical efficiency in simulations of bulk solids. We have developed a new set of PAW atomic data files for most of the stable elements in the periodic table. These files are provided in a standard XML format for use in any PAW electronic structure code. The new dataset performs well as measured by the “Δ$\Delta$” evaluation criterion introduced by Lejaeghere et al. (2014), and also performs well in a modified evaluation scheme proposed in the present paper.