Search results for "absorption spectroscopy"
showing 10 items of 828 documents
Short- and long-range order in La1−xSrxCoO3 and La1−xBaxCoO3
2008
Abstract The short- and long-range order correlations of the crystal structure in the distorted perovskites La 1− x Sr x CoO 3 and La 1− x Ba x CoO 3 (0.0⩽ x ⩽0.5) have been studied by the neutron powder diffraction (NPD) and the Co K-edge X-ray absorption spectroscopy (XAS) measurements. The results of XAS and NPD indicate a local distortion around the Co 3+ ions in LaCoO 3 at room temperature. The substitution of the La 3+ ions by the Sr 2+ (Ba 2+ ) ions leads to a gradual increase of the Co–O–Co angle and is accompanied by an increase of the mean square relative displacement (MSRD) of the Co–O bond. These results correlate with an increase of the oxygen amplitude vibration in the directi…
Localized magnetic moments in the Heusler alloy Rh2MnGe
2009
X-ray magnetic circular dichroism (XMCD) of core-level absorption (x-ray absorption spectroscopy, XAS) spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Rh2MnGe at the Rh M3,2 and Mn L3,2 edges. The ratio of Rh and Mn spin moments amounts to 0.05 which is smaller than the ratio of 0.1 determined by a local density approximation electronic band structure calculation. We have found that the orbital moments of the Rh 4d and Mn 3d states are very small. The observed Rh 2p XAS spectrum can be understood on the basis of the Rh 3d partial density of unoccupied states as is typical for metals. The observed features of the Mn 2p XAS and XMCD spectra are dominated…
Interfacial reaction between deposited molybdenum and TiO2(110) surface: role of the substrate bulk stoichiometry
2004
Abstract The interfacial reaction between deposited molybdenum and three different TiO2(1 1 0) substrates (a bulk and surface stoichiometric TiO2; a bulk stoichiometric and surface reduced crystal; a bulk and surface slightly reduced crystal) was investigated by means of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). While the interfacial reaction is not a function of the substrate at room temperature (in any case, molybdenum layers grow with oxygen dissolved in), this study clearly reveals a strong effect of the substrate bulk stoichiometry on the chemical state of the deposit after annealing up to 750 °C whereas the substrate surface stoichiome…
Study of the structural phase transformation of iron oxide nanoparticles from an Fe2+ ion source by precipitation under various synthesis parameters …
2015
Abstract Magnetite nanoparticles were precipitated from a pure aqueous ferrous salt solution in an air atmosphere. The influence of the solution molarity, the rate of precipitator agent addition, stirring time and annealing temperature was studied. The morphology, crystalline phase purity and magnetic properties of the obtained powders were studied by X-Ray powder diffraction (XRD), Scanning electron microscopy (SEM), Differential thermal analysis (DTA), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and Vibrating sample magnetometer (VSM). The synthesis conditions were seen to have an effect on phase composition. It was possible to obtain near stoichiometric Fe…
Formation of nanostructures in Eu3+ doped glass-ceramics: an XAS study.
2012
We describe the results of x-ray absorption experiments carried out to deduce structural and chemical information in Eu(3+) doped, transparent, oxyfluoride glass and nanostructured glass-ceramic samples. The spectra were measured at the Pb and Eu-L(III) edges. The Eu environment in the glass samples is observed to be similar to that of EuF(3). Complementary x-ray diffraction experiments show that thermal annealing creates β-PbF(2) type nanocrystals. X-ray absorption indicates that Eu ions act as seeds in the nanocrystal formation. There is evidence of interstitial fluorine atoms around Eu ions as well as Eu dimers. X-ray absorption at the Pb-L(III) edge shows that after the thermal treatmen…
Near-infrared luminescence of isolated and exchange-coupled Ni2+ ions in NicMg1−cO solid solutions
1997
NicMg1−cO (0.01 ⩽ c ⩽ 1) solid solutions were studied by near-infrared luminescence, optical absorption and X-ray absorption spectroscopies. It was found that Ni2+ ions form at c < 0.2 the exchange-coupled pairs, strongly bound via 90 ° super-exchange interactions, and are displaced at c ⩽ 0.6 to the ‘off-center’ positions. This explains the origin of the zero-phonon line splitting observed in the optical absorption and luminescence spectra. It was also found that the effective energy transfer from the single Ni2+ ions to the exchange-coupled Ni2+-Ni2+ pairs occurs at temperatures below 40 K.
Use of alkylarsonium directing agents for the synthesis and study of zeolites
2019
[EN] Expanding the previously known family of -onium (ammonium, phosphonium, and sulfonium) organic structure-directing agents (OSDAs) for the synthesis of zeolite MFI, a new member, the arsonium cation, is used for the first time. The new group of tetraalkylarsonium cations has allowed the synthesis of the zeolite ZSM-5 with several different chemical compositions, opening a route for the synthesis of zeolites with a new series of OSDA. Moreover, the use of As replacing N in the OSDA allows the introduction of probe atoms that facilitate the study of these molecules by powder X-ray diffraction (PXRD), solid-state nuclear magnetic resonance (MAS NMR), and X-ray absorption spectroscopy (XAS)…
A high-temperature x-ray absorption spectroscopy study of
1996
We present for the first time a high-temperature x-ray absorption spectroscopy study of at the edge. The analysis both of the XANES and of the EXAFS regions shows that, in , a progressive localization of the 5d states following the temperature rise is accompanied at T > 350 K by a distortion of the octahedra, which increases up to the decomposition temperature . The distortion leads to the lowering of the rhenium site symmetry from and is due to the electron - phonon coupling of the 5d electrons with the and phonons and to the Jahn - Teller effect caused by the rhenium electron which becomes localized at high temperature.
Redox and Catalytic Properties of Promoted NiO Catalysts for the Oxidative Dehydrogenation of Ethane
2017
[EN] NiO and metal-promoted NiO catalysts (M-NiO, with a M/(M+Ni) atomic ratio of 0.08, with M = Nb, Sn, or La) have been prepared, tested in the oxidative dehydrogenation (ODH) of ethane, and characterized by means of XRD, TPR, HRTEM, Raman, XPS, and in situ XAS (using H-2/He, air or C2H6/He mixtures). The selectivity to ethylene during the ODH of ethane decreases according to the following trend: Nb NiO Sn NiO > La NiO > NiO, whereas the catalyst reducibility (determined by both TPR and XAS using H-2/He mixtures) shows the opposite trend. However, different reducibility and catalytic behavior in the absence of oxygen (ethane/He mixtures) have been observed, especially when comparing Nb- a…
The Use of X-ray Absorption Spectra for Validation of Classical Force-Field Models
2015
Abstract Extended X-ray absorption fine structure (EXAFS) spectroscopy and molecular dynamics (MD) simulations are two complementary techniques widely used to study the atomic structure of materials. Their combined use, known as the MD-EXAFS approach, allows one to access the structural information, encoded in EXAFS, far beyond the nearest coordination shells and to validate the accuracy of the interaction potential models. In this study we demonstrate the use of the MD-EXAFS method for a validation of several force-field models on an example of the cubic-perovskite SrTiO3 and hexagonal wurtzite-type ZnO crystals.