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The sensitivity of x-ray absorption spectroscopy (XAS) to the local structure around a selected atomic species symmetry, distances and angles is commonly exploited to quantitatively describe systems where all the configurations around the absorbing atoms may be approximated by the mean one. However, in many cases of disordered systems a simple desc...
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... using two techniques, which allow to reproduce the atomic structure up to ∼3.2 Å. The first one is a model-independent approach, implemented by the EDARDF code [16]. It allows to reconstruct the total radial distribution function (RDF) without any assumption on its shape. As an example, the RDFs obtained for the glasses at 77 K are shown in Fig. ...
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We develop a multi-scale theoretical approach aimed at calculating from first-principle X-ray absorption spectra of liquid solutions and disordered systems. We test the method by considering the paradigmatic case of Zn(II) in water which, besides being relevant in itself, is also of interest for biology. With the help of classical molecular dynamic...
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Phosphate-based glasses (PBGs) are traditionally prepared using the high-temperature melt-quenching (MQ) route or via the more recent sol–gel (SG) method that requires the use of organic solvents. The coacervation method represents an excellent inexpensive and green alternative to MQ and SG, being performed in aqueous solution and at room temperature. Coacervation is particularly applicable for the production of PBGs designed for biomedical applications because it allows for the inclusion of temperature-sensitive molecules and does not require the use of toxic solvents. Whereas the atomic structure of the MQ and SGPBGs is known, the atomic structure of those prepared via coacervation has yet to be investigated. In this study, a comprehensive advanced structural characterization has been performed on phosphate-based glasses in the system P2O5–CaO–Na2O–Ag2O (Ag2O mol % = 0, 1, 3, 5, 9, and 14) prepared via the coacervation method. Glasses within this system should find application as bioresorbable biomaterials thanks to their ability to release bioactive ions in a controlled manner. In particular, they possess antibacterial properties, inferred by the release of Ag⁺ over time. High-energy X-ray diffraction (HEXRD), ³¹P and ²³Na solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and X-ray absorption Spectroscopy (XAS) at the Ag K-edge were used to probe the atomic structure of the glasses after drying in vacuum and after calcination at 300 °C. The length of the polyphosphate chains in the solid state appears to be independent of silver concentration; however, significant degradation of these chains is seen after calcination at 300 °C. Atomic-scale characterisation results indicate that the structure of these glasses is akin to that of other silver-doped phosphate glasses prepared using the MQ and SG methods. This suggests that phosphate-based glasses prepared using milder and greener conditions may have similar chemical and physical properties such as solubility, biocompatibility, and antibacterial properties.
Barium borate xBaO-(1−x)B2O3 and lithium borate xLi2O-(1−x)B2O3 glasses were investigated by x-ray absorption spectroscopy at the boron K-edge and barium LIII-edge. This is the first XAS work at the K edge of boron in lithium borate and barium borate glasses. A resonant peak at the B K-edge was observed at around 194·6 eV; its intensity and area decrease with increasing the amount of four-coordinated B changed by increasing the modifier oxide content and reach their minima at 45 mol% BaO and 40 mol% Li2O. Ba NII, III-edges were also observed. With increasing BaO content in barium borate glasses, the local structure around Ba changes from a single shell structure of six-coordinated O atoms at a Ba-O interatomic distance of 2·77 Å to a two-shell structure with distances 2·67 and 2·90 Å and coordination numbers four and 3-4, respectively.
The local structure around Ag ions in silver borate glasses g-Ag 2 O·nB 2 O 3 n =2,4 was studied by x-ray absorption spectroscopy at the Ag K edge for temperatures from 77 to 450 K. Extended x-ray absorption fine structure EXAFS analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribution functions RDFs around Ag ions were reconstructed using a method based on the direct inversion of the EXAFS expression. The RDFs consist of about eight atoms oxygens and borons, exhibit a relatively weak temperature dependence, and indicate the presence of strong static disorder. Two main components can be identified in RDFs, located at about 2.3– 2.4 Å and 2.5– 3.4 Å, respectively. The chemical types of atoms contributing to the RDF were determined via a simulation of configurationally averaged x-ray absorption near-edge structure XANES and EXAFS signals. The immediate neighborhood of Ag contains mostly oxygens while borons dominate at larger distances. The combination of EXAFS and XANES tech-niques allowed us to determine a more complete structural model than would be possible by relying solely on either EXAFS or XANES alone.
