Laurent-C. Duda

FE L2,3 LINEAR AND CIRCULAR MAGNETIC DICHROISM OF FE3O4

Abstract Measurements of circular and linear magnetic dichroism at the Fe 2p edges of Fe 3 O 4 are presented. The dichroism results from the ferrimagnetic ordering of Fe d 5 and d 6 ions in octahedral and tetrahedral sites. Atomic calculations taking into account the different crystal fields have been fitted to all L -edge spectra of this compound. A chemical shift of 1.5 eV between the dominant L 3 peak of the octahedral Fe 2+ and Fe 3+ ions gives the best fit of linear and circular dichroism. We investigate whether these parameters can be transferred to predict the Fe 2p dichroism in other ferrites.

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.

Polarized resonant inelastic X-ray scattering from single-crystal transition metal oxides

Abstract Inelastic X-ray scattering of synchrotron radiation at core resonances has turned soft X-ray emission spectroscopy into a new tool for probing the electronic structure of transition metal (TM) oxides at high resolution. In contrast to ordinary fluorescence, the ultimate achievable resolution is not restricted by the core hole life time at resonant excitation. We show that resonant inelastic X-ray scattering (RIXS) at TM oxide L-edges are a unique probe of charge-neutral crystal-field, so-called, dd-excitations. Here we discuss polarization resolved RIXS of the TM oxides La2CuO4, FeCO3, and α-Fe2O3. We find that the electronic structure at the TM ion sites may be described well by the ionic d9, d6, and d5, respectively. The experimental RIXS spectra are compared with atomic spectra using the appropriate ionic ground-state configurations and good agreement is found when the appropriate crystal symmetry is taken into account. In the case of La2CuO4 also charge transfer excitations are observed and show polarization dependence. We conclude that polarization resolved RIXS allows accurate determination of energy positions and symmetry character of charge-neutral local excitations.

Electronic structure of Li-inserted V6O13 battery cathodes: Rigid band behavior and effects of hybridization

Resonant soft x-ray emission (SXE) spectroscopy was used to study the electronic structure of LixV6O13 battery cathodes. We observe that the V 3d-bands of V6O13 exhibit a rather rigid behavior. Upon lithiation, electrons enter the top of the valence band and add intensity to the corresponding part of the V L-emission spectrum without significantly distorting the lower lying bands. We perform ab initio calculations which are in good agreement with the experimental results. Moreover, we find that lithiation leads to an overall decrease of the V 3d–O 2p hybridization. In contrast to x-ray diffraction, it is possible to study charge transfer effects in Li-batteries with SXE spectroscopy over the entire lithiation range.

Resonant soft X-ray emission spectroscopy of doped and undoped vanadium oxides

Resonant soft X-ray emission (RSXE) spectra of NaV2O5, MoxV1-xO2 and V2O3 have been recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. Resonant excitation allows us, firstly, to separate V 3d and O 2p projected density-of-states of the valence band and, secondly, to study charge-neutral low-energy excitations due to resonant inelastic X-ray scattering (RIXS). We found that both the V L- and the O K-emission spectra clearly show components originating from O 2p- and V 3d-states, reflecting the high degree of hybridization of the valence band in all compounds. At threshold excitation we observed that NaV2O5 spectra are dominated by RIXS whereas MoxV1-xO2 and V2O3 spectra show bandlike features, which may be due to differences in the correlation effects of the compounds. We compared the RSXE spectra with cluster model calculations, which gives a good account for NaV2O5 whereas the RSXE spectra of the other compounds show RIXS only at certain energies well above the threshold. In fact, we interpret the trend in the RSXE spectra of the MoxV1-xO2 compound system as a successive filling of the (rigid) V 3d band with increasing Mo content

Resonant soft X-ray emission spectroscopy of V2O3, VO2 and NaV2O5

Resonant soft X-ray emission (RSXE) spectra of V2O3, VO2 and NaV2O5 were recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. The V L- and O K-emission in these vanadium oxide bands possess considerable overlap. By resonant excitation we can tune the energy to the absorption thresholds, thereby eliminating this overlap. Hereby we obtain the V 3d and O 2p projected density-of-states of the valence band. Resonant inelastic X-ray scattering (RIXS) is found to be weak in V2O3, which we explain as being due to its metallic character at room temperature. Vanadium dioxide (VO2), semiconducting at room temperature, shows considerable RIXS features only at the O K-emission band. Distinct RIXS structures are visible in the RSXE spectra of the insulator NaV2O5. In the emission spectra excited at the V L-thresholds of this ternary vanadium oxide, dd-excitations of the V dxy subband at an energy loss of -1.7 eV are observed. Our observation, that RIXS is stronger for insulators than for metals, should be taken advantage of for studying insulator-to-metal transitions in vanadium compounds in the future.

Resonant soft X-ray emission of solids and liquids

Abstract Resonant soft X-ray emission spectroscopy (SXES) has become a powerful tool for investigating the electronic structure of a large variety of materials and systems. This is due to the vastly improved performance of third generation high brightness synchrotron radiation sources and soft X-ray emission spectroscopy dedicated beamlines. The experiments presented here were performed at two synchrotron facilities: beamline 7.0.1 at the Advanced Light Source (Berkeley, CA, USA) and beamline I5-11c at MAXLAB (Lund, Sweden). We show that resonant soft X-ray emission spectroscopy can give valuable information about the electronic structure of highly correlated materials such as cuprates and vanadates. Recently, we have succeeded in constructing soft X-ray window-sealed liquid cell containers for the study of the electronic structure of liquid state samples. Here we present some first results from salt solutions showing how information on the water–ion interaction can be extracted.

Li insertion into V6O13 battery cathodes studied by soft x-ray spectroscopies

Changes in the electronic structure of V6O13 on lithium-ion insertion into battery cathodes were studied by soft x-ray absorption (SXA) spectroscopy and resonant soft x-ray emission (SXE) spectroscopy. SXA and resonant SXE spectra were recorded ex situ for cycled battery cathodes discharged to different potentials corresponding closely to distinct lithiated stages (LixV6O13,x=0,1,…,6). Large systematic changes were observed in the vanadium and oxygen x-ray spectra, reflecting the effects of electrochemical reduction associated with the Li-ion insertion. Spectral shape analysis indicates that a large fraction of the vanadium ions have been reduced to V3+ ions for the highest degree of lithiation, x=6. Nevertheless, further lithiation may be possible, in view of the linear development of the vanadium and oxygen bands on charge uptake.

Magnetic circular dichroism in soft X-ray emission of itinerant and localized magnets

Abstract I present spin-resolved valence band studies of magnetic 3d-transition metal materials using magnetic circular dichroism in soft X-ray emission (SXEMCD). The non-resonantly excited SXEMCD of Fe, Co, and Ni, as well as several compounds with ferromagnetically aligned Mn atoms are compared. A simple model picture accounts for the basic principles involved. Quantitative estimates show that it is likely that substantial valence-band relaxation due to the intermediate-state core hole takes place in itinerant systems. A z+1 approximation shows that the magnitude of SXEMCD is proportional to the magnetic moment of such a core hole impurity. Magnetic circular dichroism in resonant inelastic X-ray scattering (RIXS) is discussed. For strongly localized 3d-electrons as found in some Mn compounds RIXS probes dd-excitations and the corresponding MCD. For more itinerant systems, such as iron, it is argued that RIXS leads to the observation of a convolution of the occupied and (parts of ) the unoccupied 3d-bands. Finally, an overview of possible future developments in helicity-resolved SXES is given.

Resonant inelastic soft X-ray scattering of insulating cuprates

We have performed high-resolution inelastic X-ray emission scattering experiments at the Cu 3p-, Cu 3s-, and O 1s-resonances of the insulating cuprates CuGeO3, CuO, La2CuO4, and SrCuO2. We introduce the novel low-energy s-edge Cu-RIXS which reveals a dd-excitation peak, which was previously unobserved due to insufficient resolution and intensity in high-energy (Cu 1s RIXS). Also, O 1s-RIXS of all cuprate sample is investigated. Surprisingly, there is a large spread in the energy loss values of the RIXS features for different compounds and we explain this by assigning the larger energy features to the occurrence of a Zhang–Rice singlet while the lower energy feature (only observed for CuGeO3) is assigned to a dd-excitation.