Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9.

The interplay between the quasi 1-dimensional CuO-chains and the 2-dimensional CuO2 planes of YBa(2)Cu(3)O(6+x) (YBCO) has been in focus for a long time. Although the CuO-chains are known to be important as charge reservoirs that enable superconductivity for a range of oxygen doping levels in YBCO, the understanding of the dynamics of its temperature-driven metal-superconductor transition (MST) remains a challenge. We present a combined study using x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) revealing how a reconstruction of the apical O(4)-derived interplanar orbitals during the MST of optimally doped YBCO leads to substantial hole-transfer from the chains into the planes, i.e. self-doping. Our ionic model calculations show that localized divalent charge-transfer configurations are expected to be abundant in the chains of YBCO. While these indeed appear in the RIXS spectra from YBCO in the normal, metallic, state, they are largely suppressed in the superconducting state and, instead, signatures of Cu trivalent charge-transfer configurations in the planes become enhanced. In the quest for understanding the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite cuprate materials, the observation of such an interplanar self-doping process in YBCO opens a unique novel channel for studying the dynamics of HTSC.

X-ray absorption spectroscopy and resonant inelastic scattering study of the first lithiation cycle of the Li-ion battery cathode Li(2-x)MnSiO4.

We employ soft X-ray absorption spectroscopy and resonant inelastic X-ray scattering spectroscopy to study the redox behavior in the first lithiation/delithiation cycle of Li(2-x)MnSiO4 (4.0-4.6 V). For extraction of lithium ions up to an end potential of 4.1 V, we do not detect any change in the oxidation state for the expected redox-active Mn atom, instead the electronic structure of the Si-O network is affected. Above 4.1 V, there is an abrupt change in the oxidation state of the Mn-ions, from 2+ to 4+, which is accompanied by a complete loss of long range order in the material, as detected by X-ray diffraction. Further lithium extraction leads to progressive loss of crystallinity of Li(2-x)MnSiO4, rather than formation of a new structure, explaining the measured first-cycle capacity loss of this material. Our results suggest that future improvement of the crystalline stability of the material, particularly with respect to the SiO4 network, is required to harness the full charge capacity of Li(2-x)MnSiO4.

Reaction cell for in situ soft x-ray absorption spectroscopy and resonant inelastic x-ray scattering measurements of heterogeneous catalysis up to 1 atm and 250 °C.

We present a novel in situ reaction cell for heterogeneous catalysis monitored in situ by x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS). The reaction can be carried out at a total pressure up to 1 atm, a regime that has not been accessible to comparable in situ techniques and thus closes the pressure gap to many industrial standard conditions. Two alternate catalyst geometries were tested: (A) a thin film evaporated directly onto an x-ray transparent membrane with a flowing reaction gas mixture behind it or (B) a powder placed behind both the membrane and a gap of flowing reaction gas mixture. To illustrate the working principle and feasibility of our reaction cell setup we have chosen ethylene epoxidation over a silver catalyst as a test case. The evolution of incorporated oxygen species was monitored by total electron/fluorescence yield O K-XAS as well as O K-RIXS, which is a powerful method to separate contributions from inequivalent sites. We find that our method can reliably detect transient species that exist during catalytic reaction conditions that are hardly accessible using other spectroscopic methods.

Two-spinon and orbital excitations of the spin-Peierls system TiOCl.

We combine high-resolution resonant inelastic x-ray scattering with cluster calculations utilizing a recently derived effective magnetic scattering operator to analyze the polarization, excitation energy, and momentum-dependent excitation spectrum of the low-dimensional quantum magnet TiOCl in the range expected for orbital and magnetic excitations (0-2.5 eV). Ti 3d orbital excitations yield complete information on the temperature-dependent crystal-field splitting. In the spin-Peierls phase we observe a dispersive two-spinon excitation and estimate the inter- and intradimer magnetic exchange coupling from a comparison to cluster calculations.

Resonant inelastic X-ray scattering and X-ray absorption spectroscopy on the cathode materials LiMnPO4 and LiMn0.9Fe0.1PO4--a comparative study.

We present a study of the charge-state behavior of the Li-ion battery cathode materials Li(x)MnPO(4) and Li(x)Mn(0.9)Fe(0.1)PO(4) using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). A set of six identical battery cathodes for each material have been cycled and left in different charge states in the range of x = 0.2…1.0 before disassembly in an Ar glove box. Unexpectedly, we find that the Mn 3d-bands are almost inert to the delithiation process, suggesting that Mn ions participate to a very small extent in the charge compensation process. In Li(x)Mn(0.9)Fe(0.1)PO(4) the Fe 3d-band shows much more response to delithiation than the Mn 3d-band. The O 2p-band hybridizes with the bands of the other ions in Li(x)MnPO(4) and Li(x)Mn(0.9)Fe(0.1)PO(4) and thus, indirectly, carries useful information about the effects of delithiation at all ion sites. We conclude that the redox reactions during lithiation/delithiation of these materials are complex and involve repopulation of charges for all constituent elements.

Resonant inelastic X-ray scattering and X-ray absorption spectroscopy on the negative electrode material Li0.5Ni0.25TiOPO4 in a Li-ion battery.

We have studied the first lithiation/delithiation cycle of the Li-ion battery electrode material Li(x)Ni(0.25)TiOPO(4) applying X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). A set of ten identical Li(x)Ni(0.25)TiOPO(4) battery electrodes have been cycled and left in different states of charge in the range of x = 0.5 … 2.5, before disassembly in an Ar filled glove box. We find that Ni-, Ti-, and O-ions are affected simultaneously, rather than sequentially, upon lithiation of the material. In particular, Ni is reduced from Ni(2+) to Ni(0) but only partially re-oxidized to Ni(1+), again, by delithiation. Overall, there is considerable "crosstalk" between the different atomic species and non-linearity in the response of the electronic structure during the lithiation/delithiation process. Fortuitously, the background variation in Ni L-XAS shows to contain valuable information about solid-electrolyte interface (SEI) creation, showing that the SEI is a function of the degree of lithiation.

A versatile soft X-ray transmission system for time resolved in situ microscopy with chemical contrast.

We present the design and operation of a versatile soft X-ray transmission system for time resolved in situ microscopy with chemical contrast. The utility of the setup is demonstrated by results from following a corrosion process of iron in saline environment, subjected to a controlled humid atmosphere. The system includes a transmission flow-cell reactor that allows for in situ microscopic probing with soft X-rays. We employ a full field technique by using a nearly collimated X-ray beam that produces an unmagnified projection of the transmitted soft X-rays (below 1.1 keV) which is magnified and recorded by an optical CCD camera. Time lapse series with chemical contrast allow us to follow and interpret the chemical processes in detail. The obtainable lateral resolution is a few mum, sufficient to detect filiform corrosion on iron.

System for in situ studies of atmospheric corrosion of metal films using soft x-ray spectroscopy and quartz crystal microbalance.

We present a versatile chamber ("atmospheric corrosion cell") for soft x-ray absorption/emission spectroscopy of metal surfaces in a corrosive atmosphere allowing novel in situ electronic structure studies. Synchrotron x rays passing through a thin window separating the corrosion cell interior from a beamline vacuum chamber probe a metal film deposited on a quartz crystal microbalance (QCM) or on the inside of the window. We present some initial results on chloride induced corrosion of iron surfaces in humidified synthetic air. By simultaneous recording of QCM signal and soft x-ray emission from the corroding sample, correlation between mass changes and variations in spectral features is facilitated.

Resonant inelastic x-ray scattering at the oxygen K resonance of NiO: nonlocal charge transfer and double-singlet excitations.

We report high-resolution polarization-dependent resonant inelastic x-ray scattering (RIXS) at the O K resonance of NiO showing a rich excitation spectrum. We perform multisite Ni6O19 cluster model calculations, revealing that solid state effects are substantial. We identify a nonlocal charge transfer excitation at 4-5 eV and double-singlet creation at 1.75 eV, both exhibiting significant scattering geometry dependence. Apart from an intense band of local charge transfer excitations (above 5 eV) also dd excitations at 1 eV are observed. Finally, we point out that O K RIXS of correlated metal oxides allows a quantitative and consistent determination of the charge transfer energy delta and the Hund coupling energy J(H).