François Baudelet

ODE: a new beam line for high-pressure XAS and XMCD studies at SOLEIL

X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) under pressure are probes of local order and microscopic magnetic properties. XMCD is a selective probe that has access to a large variety of elements. The dispersive extended X-ray absorption fine structure (EXAFS) station at SOLEIL (ODE beam line) provides the possibility to perform numerous pressure XAS and XMCD experiments with an excellent statistic. The main advantages of dispersive XAFS are the focusing optics, the short acquisition time (few μs) and great stability during the measurements due to the absence of any mechanical movement. These advantages allow the study of small samples, 70 μm at SOLEIL, which is mandatory in the case of high-pressure studies. We present the new ODE beam line at SOLEIL and its first high-pressure XMCD results.

XMCD under pressure at the Fe K edge on the energy-dispersive beamline of the ESRF.

The present paper demonstrates the feasibility of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at high pressure at the Fe K edge on the ID24 energy-dispersive beamline of the ESRF. In 3d transition metals, performing experiments at the hard X-ray K edge rather than at the magnetically interesting soft X-ray L edges represents the only way to access the high-pressure regime obtainable with diamond anvil cells. The simultaneous availability of a local structure (XAS) and of a magnetic (XMCD) probe on the sample under identical thermodynamical conditions is essential for studying correlations between local structural and magnetic properties. The state-of-the-art theoretical understanding of K-edge XMCD data is briefly summarized, and the set-up of beamline ID24 for high-pressure XMCD experiments is illustrated and the conditions required to perform measurements at the K edges of 3d transition metals are underlined. Finally, two examples of recent high-pressure results at the Fe K edge in pure Fe and Fe3O4 powder are presented.

Emergence of ferromagnetism and Jahn-Teller distortion in LaMn1−xCrxO3 (x < 0.15)

The emergence of a ferromagnetic component in LaMnO3 with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr K edges. The local magnetic moment strength for the Mn and Cr are proportional to each other and follows the macroscopic magnetization. The net ferromagnetic components of Cr3+ and Mn3+ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of Cr3+ ions up to x = 0.15 does not decrease the Jahn-Teller (JT) distortion and consequently does not signi cantly a ect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.

Photo-Induced Electron Transfer and Magnetic Switching in CoFe Cyanides: Study of the Metastable State

Abstract Photo-induced magnetisation has been recently evidenced by Hashimoto et al. in CoFe cyanide[1]. We synthesized a diamagnetic RbFeCo cyanide able to present the same photomagnetic effect. The magnetic properties of the excited state, the electronic structure and the local structure of the ground and the excited states have been investigated. The conditions required to observe the phenomenon and the mechanism of the electron transfer are discussed.

Room-temperature photoinduced electron transfer in a Prussian blue analogue under hydrostatic pressure.

In from the cold: The Co(III)Fe(II) state of a CoFe Prussian blue analogue undergoes a Co(III)-Fe(II) →(Co(II)-Fe(III))* electron transfer at room temperature when irradiated by visible light (532 nm; see scheme). This property was confirmed using energy-dispersive X-ray absorption spectroscopy at the Co and Fe K-edges of the piezo-induced Co(III)Fe(II) state.

High pressure x-ray diffraction and extended x-ray absorption fine structure studies on ternary alloy Zn1−xBexSe

The ternary semiconductor alloy Zn1−xBexSe has been studied under high pressure by x-ray diffraction and extended x-ray absorption fine structure (EXAFS) at the Zn and Se K-edge in order to determine the bulk and bond-specific elastic properties. Our measurements on samples with x=0.06–0.55 show pressure induced phase transformation from zinc blende to NaCl. The phase transformation pressure increases linearly with x. Murnaghan equation of state fitting to the data yields the unit cell volume at ambient pressure and bulk modulus, both of which follow the Vegard’s law. Nearest neighbor bond distances derived from EXAFS do not show sharp phase transition except for x=0.06. Bond modulus derived for the Zn–Se bonds shows them to be apparently stiffer than the bulk alloy, which is nontrivial. This tendency increases with increasing x and a strong positive bowing from the Vegard’s law is observed. We attribute the observed anomalies to the contrastingly different properties of the two components ZnSe and BeSe.

Temperature dependence of the pre-edge structure in the Ti K-edge x-ray absorption spectrum of rutile

The temperature dependence of the pre-edge features in x-ray absorption spectroscopy is reviewed. Then, the temperature dependence of the pre-edge structure at the K-edge of titanium in rutile TiO(2) is measured at low and room temperature. The first two peaks grow with temperature. The fact that these two peaks also correspond to electric quadrupole transitions is explained by a recently proposed theory.

Multi-spectroscopic monitoring of cisplatin-derived species delivery from ureasil polyether hybrid matrix

Driven by the challenges involved in the development of new advanced materials with unusual drug delivery profiles capable of improving the therapeutic and toxicological properties of existing cancer chemotherapy, the one-pot sol–gel synthesis of flexible, transparent and insoluble urea-cross-linked polyether–siloxane hybrids has been recently developed. In this one-pot synthesis, the strong interaction between the antitumor cisplatin (CisPt) molecules and the ureasil–poly(propylene oxide) (PPO) hybrid matrix gives rise to the incorporation and release of an unknown CisPt-derived species, hindering the quantitative determination of the drug release pattern from the conventional UV-Vis absorption technique. In this article, we report the use of an original synchrotron radiation calibration method based on the combination of XAS and UV-Vis for the quantitative determination of the amount of Pt-based molecules released in water. Thanks to the combination of UV-Vis, XAS and Raman techniques, we demonstrated that both the CisPt molecules and the CisPt-derived species are loaded into an ureasil–PPO/ureasil–poly(ethylene oxide) (PEO) hybrid blend matrix. The experimentally determined molar extinction coefficient of the CisPt-derived species loaded into ureasil–PPO hybrid matrix enabled the simultaneous time-resolved monitoring of each Pt species released from this hybrid blend matrix.

Mosaic crystals as X-ray phase plates

Abstract It is shown that a beryllium crystal whose mosaicity is of the order of 80 arcsec can be used as an efficient X-ray quarter-wave plate. The experiment is realized on the energy dispersive absorption spectrometer at LURE (Orsay, France), where X-ray circular magnetic dichroic spectra, proportional to the circular polarization rate, are recorded.

New scientific opportunities for high pressure research by energy-dispersive XMCD

ABSTRACT XMCD under pressure is used to study the magnetic properties of the transition metal (TM) systems for over 15 years. We present the technique and how it has been developed. The energy dispersive XAS spectrometer is particularly suited for these studies. The effect of pressure on TM magnetism is discussed. Recent studies performed at different edges illustrate the information that can be obtained through XMCD. Finally, some results obtained on TMs are presented, either at the LII,III edges of 5d metals or at the K edge of 3d metals, which correspond to the energy ranges that can be probed when using diamond anvil cells for high pressure. Different cases are treated: pure 3d metals, alloys, magnetic insulator and inorganic compounds.