No abstract

SnO2 gas sensors with palladium as additive in the range of 0.2 wt% and 3 wt% were studied by in situ X-ray absorption spectroscopy under idealized and real operating conditions. Simultaneously to the structural studies, measurements of the sensing properties were undertaken allowing for the determination of structure-function relationships. For this purpose a new in situ spectroscopic cell was designed which permitted on the one hand sensing on conventional screen printed 50 microm thick sensing layers and on the other hand structural analysis with X-rays provided by an insertion device at a 3rd generation synchrotron facility in fluorescence mode. Pd K-edge XANES and EXAFS results on gas sensors showed that palladium, present in an oxidized state, is finely dispersed if it is added in small quantities (0.2 wt%) while it forms clusters at higher concentrations (3 wt%). This is also reflected by the much easier reduction of palladium in the latter, higher concentrated ones. Under realistic sensing conditions (30-200 ppm H2; 10-50 ppm CO in dry and humid air at 200 and 300 degrees C) for the low additive concentration samples, no change in oxidation state was observed, i.e. palladium remained in its oxidized state. This has important consequences on the understanding and modeling of the gas sensing mechanism.

High-resolution x-ray absorption measurements reveal a rare-earth-dependent splitting of the Ni K edge in the insulating, charge-disproportionated state of the whole RNiO 3 perovskite family. The splitting is five times larger for LuNiO 3 ͓2.5͑1͒ eV͔ than for PrNiO 3 ͓0.5͑3͒ eV͔, suggesting that the charge transfer between Ni 3+␦ and Ni 3−␦ decreases by approaching the itinerant limit and is larger for the heavier lanthanides than suggested in previous studies. The spectroscopic signature of the two Ni sites remains visible above the metal-insulator transition, in agreement with the persistence of dynamic Ni 3+␦ / Ni 3−␦ charge fluctuations in the metallic phase. This last result generalizes the occurrence of charge disproportionation as alternative to Jahn-Teller distortions to the dynamic regime, giving further support to recent theoretical work ͓I.

and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

The changes in the electronic structure of V2O3 across the metal-insulator transition induced by temperature, doping, and pressure are identified using high resolution x-ray absorption spectroscopy at the V pre-K edge. Contrary to what has been taken for granted so far, the metallic phase reached under pressure is shown to differ from the one obtained by changing doping or temperature. Using a novel computational scheme, we relate this effect to the role and occupancy of the a{1g} orbitals. This finding unveils the inequivalence of different routes across the Mott transition in V2O3.

Valence-to-core X-ray emission spectroscopy was applied to study a composition of chromium coatings electrodeposited from Cr(III) sulfate electrolytes with the addition of formic or oxalic acid. It was shown that the obtained crystallographically amorphous deposits contain chromium carbide compounds. These results indicate that nanodimensional Cr crystallites formed during the electrodeposition process are characterized by very high electrocatalytic activity.

No abstract

Spin and orbital magnetic moments of rhenium in AAЈFeReO 6 double perovskites ͑A , AЈ = Ba, Sr, and Ca͒ have been directly probed employing x-ray magnetic circular dichroism at the Re L 2,3 edges. A considerable orbital magnetic moment is observed in all the compounds studied, which confirm theoretical predictions of unquenched Re orbital moment despite its octahedral coordination. Relative orbital-to-spin moment ratio alters with lattice distortion from m L / m S = −0.285 to − 0.337 from Ba 2 FeReO 6 to Ca 2 FeReO 6 , respectively. Moreover, the spin moment of Re ion scales with Curie temperature, the most relevant property in spin electronics application of the compounds studied. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2234292͔ Ordered double perovskites have recently attracted great interest due to their large spin polarization and Curie temperature ͑T C ͒ much higher than room temperature. These properties are strongly desired in order to realize reasonable magnetoresistance effects at room temperature, which is not only a challenging subject of fundamental science but also an important phenomenon for potential applications in spin electronics. Therefore the first observation of substantial magnetoresistance at room temperature in Sr 2 FeMoO 6 ͑Ref. 1͒ quickly led to production of magnetic tunnel junctions and magnetoresistive potentiometers.Currently, the other ordered double perovskites AAЈBBЈO 6 ͑A , AЈ = Ca, Sr, Ba, La, etc.; BBЈ = FeMo, FeRe, CrRe, CrW, etc.͒ are being intensively studied in order to find a material with optimal performance. 2-7 Among them the Re-based double perovskites are the most promising compounds in terms of high Curie temperature, e.g., 538 and 635 K in Ca 2 FeReO 6 and Sr 2 CrReO 6 , respectively.3 Moreover, Re-based double perovskites are magnetically hard 4,6,8 and reveal large magnetoelastic effects, 9 which can only be explained by a substantial magnetocrystalline anisotropy due to the anisotropy of an unquenched orbital moment of Re.