Potassium doping effect on the lattice softening and electronic structure of Ba(1-x)K(x)Fe(2)As(2) probed by X-ray absorption spectroscopy

Ba(1-x)K(x)Fe(2)As(2) superconducting samples (x = 0, 0.2, 0.4, 0.5) were synthesized by the solid-state reaction method. In this contribution the doping effect of potassium on the lattice dynamics in this newly discovered Ba(1-x)K(x)Fe(2)As(2) superconductor has been investigated by extended X-ray absorption fine-structure spectroscopy. The analysis shows that with potassium doping an increased disorder in the iron layers is mainly related to the softening of the Fe-Fe bond. Information about the electronic structure of these materials has also been obtained by looking at the X-ray absorption near-edge structure spectra that point out the presence of holes in the Fe-3d/As-4p hybridized orbital of the BaFe(2)As(2)-based system.     

Electronic structure effects on B K‐edge XANES of minerals

In order to assess the usability of X‐ray absorption near‐edge structure (XANES) for studying the structure of BO_n‐containing materials, the dependence of theoretical XANES at the B K‐edge on the way the scattering potential is constructed is investigated. Real‐space multiple‐scattering calculations are performed for self‐consistent and non‐self‐consistent potentials and for different ways of dealing with the core hole. It is found that in order to reproduce the principal XANES features it is sufficient to use a non‐self‐consistent potential with a relaxed and screened core hole. Employing theoretical modelling of XANES for studying the structure of boron‐containing glasses is thus possible. The core hole affects the spectrum significantly, especially in the pre‐edge region. In contrast to minerals, B K‐edge XANES of BPO₄ can be reproduced only if a self‐consistent potential is employed.     

A Li K‐edge XANES study of salts and minerals

The first comprehensive Li K‐edge XANES study of a varied suite of Li‐bearing minerals is presented. Drastic changes in the bonding environment for lithium are demonstrated and this can be monitored using the position and intensity of the main Li K‐absorption edge. The complex silicates confirm the assignment of the absorption edge to be a convolution of triply degenerate p‐like states as previously proposed for simple lithium compounds. The Li K‐edge position depends on the electronegativity of the element to which it is bound. The intensity of the first peak varies depending on the existence of a 2p electron and can be used to evaluate the degree of ionicity of the bond. The presence of a 2p electron results in a weak first‐peak intensity. The maximum intensity of the absorption edge shifts to lower energy with increasing SiO₂ content for the lithium aluminosilicate minerals. The bond length distortion of the lithium aluminosilicates decreases with increasing SiO₂ content, thus increased distortion leads to an increase in edge energy which measures lithium's electron affinity.     

Defect engineering by synchrotron radiation X‐rays in CeO2 nanocrystals

This work reports an unconventional defect engineering approach using synchrotron‐radiation‐based X‐rays on ceria nanocrystal catalysts of particle sizes 4.4–10.6 nm. The generation of a large number of oxygen‐vacancy defects (OVDs), and therefore an effective reduction of cations, has been found in CeO₂ catalytic materials bombarded by high‐intensity synchrotron X‐ray beams of beam size 1.5 mm × 0.5 mm, photon energies of 5.5–7.8 keV and photon fluxes up to 1.53 × 10¹² photons s^?1. The experimentally observed cation reduction was theoretically explained by a first‐principles formation‐energy calculation for oxygen vacancy defects. The results clearly indicate that OVD formation is mainly a result of X‐ray‐excited core holes that give rise to valence holes through electron down conversion in the material. Thermal annealing and subvalent Y‐doping were also employed to modulate the efficiency of oxygen escape, providing extra control on the X‐ray‐induced OVD generating process. Both the core‐hole‐dominated bond breaking and oxygen escape mechanisms play pivotal roles for efficient OVD formation. This X‐ray irradiation approach, as an alternative defect engineering method, can be applied to a wide variety of nanostructured materials for physical‐property modification.     

High-Porosity Metal-Organic Framework Glasses

We report a synthetic strategy to link titanium-oxo (Ti-oxo) clusters into metal-organic framework (MOF) glasses with high porosity though the carboxylate linkage. A new series of MOF glasses was synthesized by evaporation of solution containing Ti-oxo clusters Ti₁₆O₁₆(OEt)₃₂, linkers, and m-cresol. The formation of carboxylate linkages between the Ti-oxo clusters and the carboxylate linkers was confirmed by Fourier-transform infrared (FT-IR) spectroscopy. The structural integrity of the Ti-oxo clusters within the glasses was evidenced by both X-ray absorption near edge structure (XANES) and ¹⁷O magic-angle spinning (MAS) NMR. After ligand exchange and activation, the fumarate-linked MOF glass, termed Ti-Fum, showed a N₂ Brunauer–Emmett–Teller (BET) surface areas of 923 m² g⁻¹, nearly three times as high as the phenolate-linked MOF glass with the highest BET surface area prior to this report.