XANES

About: XANES is a research topic. Over the lifetime, 7737 publications have been published within this topic receiving 188032 citations.

Silicon carbonitride ceramics derived from polysilazanes Part I. Investigation of compositional and structural properties

Abstract: In this paper we report on the compositional and structural properties of ceramics in the Si–C–N system The amorphous material synthesised by thermally induced ceramisation of an organo-substituted polysilazane crystallises at temperatures exceeding 1450°C in nitrogen atmosphere Chemical composition, solid state structure, microstructure as well as the crystallisation behaviour depend on: (i) the synthesis conditions; (ii) the pyrolysis atmosphere; and (iii) the time and temperature of a final heat treatment The chemical composition was determined by chemical bulk analysis together with 15N-method and electron energy-loss spectroscopy (EELS) Reaction products formed during the heat treatment were analysed in-situ by thermal gravimetric analysis coupled with mass spectrometry The hybridisation of carbon in the solid reaction product was determined by X-ray absorption near-edge structure (XANES) The resulting amorphous microstructure was investigated by electron microscopy (HRTEM), while the crystalline phases were analysed by X-ray diffraction (XRD)

An experimental determination of the effect of pressure on the Fe3+/∑Fe ratio of an anhydrous silicate melt to 3.0 GPa

Abstract: The effect of pressure on the Fe 3+ /∑Fe ratio of an anhydrous andesitic melt was determined from 04 to 30 GPa at 1400 °C with oxygen fugacity controlled internally by the Ru + RuO 2 buffer Values of Fe 3+ /∑Fe were determined by Mossbauer spectroscopy on quenched glasses with a precision of ±001, one standard deviation This precision was verified independently by XANES spectroscopy of the same samples The XANES spectra show a systematic increase in energy and decrease in intensity of the 1s → 3d transition with increasing pressure The results to 20 GPa are in good agreement with predictions from density and compressibility measurements fitted to a Murnaghan equation of state, but the datum at 30 GPa has higher Fe 3+ /∑Fe than predicted from the trend established by the lower-pressure data This might be due to a coordination change in Fe 3+ at high pressure; although there is no evidence for this in the Mossbauer spectra, such a change could account for the change in intensity of the 1s → 3d transition in the XANES spectra with pressure

Redox‐Driven Migration of Copper Ions in the Cu‐CHA Zeolite as Shown by the In Situ PXRD/XANES Technique

Abstract: Using quasi-simultaneous insitu PXRD and XANES, the direct correlation between the oxidation state of Cu ions in the commercially relevant deNOx NH3-SCR zeolite catalyst Cu-CHA and the Cu ion migration in the zeolitic pores was revealed during catalytic activation experiments. A comparison with recent reports further reveals the high sensitivity of the redox-active centers concerning heating rates, temperature, and gas environment during catalytic activation. Previously, Cu+ was confirmed present only in the 6R. Results verify a novel 8R monovalent Cu site, an eventually large Cu+ presence upon heating to high temperatures in oxidative conditions, and demonstrate the unique potential in combining insitu PXRD and XANES techniques, with which both oxidation state and structural location of the redox-active centers in the zeolite framework could be tracked. (Less)

Influence of point defects on the near edge structure of hexagonal boron nitride

Abstract: Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor with applications including gate insulation layers in graphene transistors, far-ultraviolet light emitting devices and as hydrogen storage media. Due to its complex microstructure, defects in hBN are challenging to identify. Here, we combine x-ray absorption near edge structure (XANES) spectroscopy with ab initio theoretical modeling to identify energetically favorable defects. Following annealing of hBN samples in vacuum and oxygen, the B and N $K$ edges exhibited angular-dependent peak modifications consistent with in-plane defects. Theoretical calculations showed that the energetically favorable defects all produce signature features in XANES. Comparing these calculations with experiments, the principle defects were attributed to substitutional oxygen at the nitrogen site, substitutional carbon at the boron site, and hydrogen passivated boron vacancies. Hydrogen passivation of defects was found to significantly affect the formation energies, electronic states, and XANES. In the B $K$ edge, multiple peaks above the major $1s$ to ${\ensuremath{\pi}}^{*}$ peak occur as a result of these defects and the hydrogen passivated boron vacancy produces the frequently observed doublet in the $1s$ to ${\ensuremath{\sigma}}^{*}$ transition. While the N $K$ edge is less sensitive to defects, features attributable to substitutional C at the B site were observed. This defect was also calculated to have mid-gap states in its band structure that may be responsible for the 4.1-eV ultraviolet emission frequently observed from this material.