No abstract

Hydroxyapatite (HAp) is an important component of mammal bones and teeth, being widely used in prosthetic implants. Despite the importance of HAp in medicine, several promising applications involving this material (e.g., in photo-catalysis) depend on how well we understand its fundamental properties. Among the ones that are either unknown or not known accurately, we have the electronic band structure and all that relates to it, including the bandgap width. We employ state-of-the-art methodologies, including density hybrid-functional theory and many-body perturbation theory within the dynamically screened single-particle Green's function approximation, to look at the optoelectronic properties of HAp. These methods are also applied to the calculation of defect levels. We find that the use of a mix of (semi-)local and exact exchange in the exchange-correlation functional brings a drastic improvement to the band structure. Important side effects include improvements in the description of dielectric and optical properties not only involving conduction band (excited) states but also the valence. We find that the highly dispersive conduction band bottom of HAp originates from anti-bonding σ* states along the ⋯OH-OH-⋯ infinite chain, suggesting the formation of a conductive 1D-ice phase. The choice of the exchange-correlation treatment to the calculation of defect levels was also investigated by using the OH-vacancy as a testing model. We find that donor and acceptor transitions obtained within semi-local density functional theory (DFT) differ from those of hybrid-DFT by almost 2 eV. Such a large discrepancy emphasizes the importance of using a high-quality description of the electron-electron interactions in the calculation of electronic and optical transitions of defects in HAp.

Hydride formation in palladium nanoparticles was studied by Pd K-edge X-ray absorption spectroscopy in both the near-edge (XANES) and the extended (EXAFS) regions and by X-ray diffraction (XRD) both in situ as a function of temperature and hydrogen pressure. In contrast to EXAFS and XRD, which probe Pd− Pd interatomic distance changes, the direct effect of hydrogen concentration on the electronic palladium structure is observed in the intensities and the peak positions in the XANES region. By using theoretical simulations, we propose a simple analysis of hydrogen concentration based on the changes of relative peak amplitudes in the XANES region, which correlate with interatomic distance changes determined by both EXAFS and XRD. By the quantitative analysis of XANES difference spectra, we have developed a scheme to determine the hydrogen concentration in palladium nanoparticles without applying any additional calibration procedures with alternative experimental techniques.

We report an in situ, temperature and H 2 pressuredependent, characterization of (2.6 ± 0.4) nm palladium nanoparticles supported on active carbon during the process of hydride phase formation. For the first time the core−shell structure is highlighted in the single-component particles on the basis of a different atomic structure and electronic configurations in the inner "core" and surface "shell" regions. The atomic structure of these particles is examined by combined X-ray powder diffraction (XRPD), which is sensitive to the crystalline core region of the nanoparticles, and by first shell analysis of extended Xray absorption fine structure (EXAFS) spectra, which reflects the averaged structure of both the core and the more disordered shell. In the whole temperature range (0−85 °C), XRPD analysis confirms the existence of two well-separated αand β-hydride phases with the characteristic flat plateau in the phase transition region of the pressure-lattice parameter isotherms. In contrast, first shell interatomic distances obtained from EXAFS exhibit a slope in the phase transition region, typical for nanostructured palladium. Such difference is explained by distinct properties of bulk "core" which has crystalline structure and sharp phase transition, and surface "shell" which is amorphous and absorbs hydrogen gradually without forming distinguishable αand β-phases. Combining EXAFS and XRPD we extract, for the first time, the Pd−Pd first-shell distance in the amorphous shell of the nanoparticles, that is significantly shorter than in the bulk core and relevant in catalysis. The core/shell model is supported by the EXAFS analysis of the higher shells, in the frame of the multiple scattering theory, showing that the evolution of the third shell distance (ΔR 3 /R 3 ) is comparable to the evolution of (Δa/a) obtained from XRPD since amorphous PdH x shell gives a negligible contribution in this range of distances. This operando structural information is relevant for the understanding of structure-sensitive reactions. Additionally, we demonstrate the differences in the evolution of the thermal parameters obtained from EXAFS and XRPD along the hydride phase formation.

The method for quantitative determination of interatomic distances and coordination numbers ͑CN͒ in minerals and amorphous compounds by their x-ray absorption near edge structure ͑XANES͒ is proposed. The generation of the method and the proof of its main points are based on the revealed theoretical description of XANES for crystalline compounds and minerals as a sum of different photoelectron scattering processes on two-and approximately linear three-atoms chains, originated at the absorbing atom. The method consists of the Fourier filtration ͑FF͒ of the experimental XANES within the short range of photoelectron's wave numbers and the following fitting, with three varied parameters, of the first shell term extracted by the FF procedure. The accuracy of the obtained local structural parameters is illustrated for the reference crystalline minerals, which are used also to simulate the XANES formation in glasses. The application of the method to Mg XANES in diopside-glass and its crystalline equivalent, Diopside, permits one to determine the Mg-O distance and CN for the first oxygen shell around the Mg atom and to reveal the change of these parameters between the glass and the crystal with similar chemical composition.