High-resolution x-ray absorption measurements reveal a rare-earth-dependent splitting of the $\text{Ni}K$ edge in the insulating, charge-disproportionated state of the whole $R{\text{NiO}}_3$ perovskite family. The splitting is five times larger for ${\text{LuNiO}}_3$ [2.5(1) eV] than for ${\text{PrNiO}}_3$ [0.5(3) eV], suggesting that the charge transfer between ${\text{Ni}}^{3+\delta }$ and ${\text{Ni}}^{3-\delta }$ 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 ${\text{Ni}}^{3+\delta }/{\text{Ni}}^{3-\delta }$ 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. I. Mazin, D. I. Khomskii, R. Lengsdorf, J. A. Alonso, W. G. Marshall, R. M. Ibberson, A. Podlesnyak, M. J. Martínez-Lope, and M. M. Abd-Elmeguid, Phys. Rev. Lett. 98, 176406 (2007)].

SnO₂ gas sensors with palladium as additive in the range of 0.2 wt% and 3 wt% were studied by in situX-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 μm 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 °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.

Measurement of the Ce valence in the heavy fermion CeCu2Si2 is reported for the first time under pressure and at low temperature (T=14\,\,K) in proximity of the superconducting region. CeCu2Si2 is considered as a strong candidate for a new type of pairing mechanism related to critical valence fluctuations which could set in at high pressure in the vicinity of the second superconducting dome. A quantitative estimate of the valence in this pressure region was achieved from the measurements of the Ce L3 edge in the high-resolution partial-fluorescence yield mode and subsequent analysis of the spectra within the Anderson impurity model. While a clear increase of the Ce valence is found, the weak electron transfer and the continuous valence change under pressure suggests a crossover regime with the hypothetical valence line terminating at a critical end point Tcr close to zero.