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High pressure energy-dispersive X-ray absorption spectroscopy is a valuable structural technique, especially, when combined with a nano-polycrystalline diamond anvil cell. Here we present recent results obtained using the dispersive setup of the ODE beamline at SOLEIL synchrotron. The effect of pressure and temperature on the X-ray induced photoreduction is discussed on the example of nanocrystalline CuO. The possibility to follow local environment changes during pressure-induced phase transitions is demonstrated for α-MoO 3 based on the reverse Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

This work reports an X-ray diffraction, X-ray absorption, and Raman spectroscopy study of [(CH3)4N]2MnX4 (X = Cl, Br) under pressure. We show that both compounds share a similar phase diagram with pressure. A P21/c monoclinic structure describes precisely the [(CH3)4N]2MnCl4 crystal in the 0.1-6 GPa range, prior to crystal decomposition and amorphization, while [(CH3)4N]2MnBr4 can be described by a Pmcn orthorhombic structure in its stability pressure range of 0-3 GPa. These materials are attractive systems for pressure studies since they are readily compressible through the weak interaction between organic/inorganic [(CH3)4N+/MnX42-] tetrahedra through hydrogen bonds and contrast with the small compressibility of both tetrahedra. Here we determine the equation-of-state (EOS) of each crystal and compare it with the corresponding local EOS of the MnX42- and (CH3)4N+ tetrahedra, the compressibility of which is an order and 2 orders of magnitude smaller than the crystal compressibility, respectively, in both chloride and bromide. The variations of the Mn-Cl bond distance obtained by extended X-ray absorption fine structure and the frequency of the totally symmetric v1(A1) Raman mode of MnCl42- with pressure in [(CH3)4N]2MnCl4 allowed us to determine the associated Grüneisen parameter (γloc = 1.15) and hence an accurate local EOS. On the basis of a local compressibility model, we obtained the Grüneisen parameters and corresponding variations of the intramolecular Mn-Br and C-N bond distances of MnBr42- (γloc = 1.45) and (CH3)4N+ (γloc = 3.0) in [(CH3)4N]2MnBr4. [ABSTRACT FROM AUTHOR]

This work investigates the Mn2+ electronic structure and exciton dynamics in one-dimensional (1D) N(CH3)4MnCl3 (TMMC) through time-resolved excitation/emission spectroscopy and absorption measurements in the 0-10 GPa pressure range for different Cu2+ doping concentrations. The local and crystal structures have been analyzed by Raman spectroscopy and x-ray absorption measurements at the Mn K edge showing that the 1D chain structure is maintained in the whole explored pressure range. We show that both the first Mn2+ absorption band, 4T1(G), and its associated emission band experience very large pressure redshifts, which are associated with the crystal anisotropy providing large axial ligand fields at the Mn2+ site that increase with pressure. The red emission at 633 nm shows a large pressure variation of 22 nm/GPa (50 meV/GPa) making TMMC a suitable probe for using as a photoluminescence (PL) pressure gauge in the low-pressure regime. The energy-transfer exciton dynamics and trapping at non-PL centers have been explained through changes of the intrachain Mn-Mn exchange interaction and Cu2+-trap concentration carried out by applying pressure and doping, respectively. The model demonstrates that an increase of exchange interaction favors both the pumping capability and energy transfer yielding exciton migration. Under these conditions, we show that pressure enhances the PL efficiency of TMMC provided that the Cu2+ concentration responsible for the PL quenching is below 0.001 mol %. However, between 0.001% and 0.1%, the PL intensity reduces with pressure, and above 0.1%, the PL is practically quenched even at ambient conditions. [ABSTRACT FROM AUTHOR]

This work investigates the electronic structure and photoluminescence (PL) of Co2+-doped MgAl2O4 and their pressure dependence by time-resolved spectroscopy. The variations of the visible absorption band and its associated emission at 663 nm (τ = 130 ns at ambient conditions) with pressure/temperature can be explained on the basis of a configurational energy model. It provides an interpretation for both the electronic structure and the excited-state phenomena yielding photoluminescence emission and die subsequent quenching. We show that there is ah excited-state crossover (ESCO) [4T1(P) ↔2E(G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 K and 100 K to a narrow-line emission at lower temperatures. Contrary to expectations from the Tanabe-Sugano diagram, instead of enhancing ESCO phenomena, pressure reduces PL and even suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced nonradiative relaxation to lower excited states: 2E(G) → 4T1(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation. [ABSTRACT FROM AUTHOR]

This work investigates the Mn2+ electronic structure and photoluminescence (PL) in [(CH3)4N]2MnCl4 and [(CH3)4N]2MnBr4 and their variation with pressure by time-resolved excitation and emission spectra in the 0-10 GPa range. We show that both the crystal-field excitation and corresponding PL bands experience large-pressure redshifts (about 5 nm/GPa), which are associated with the high crystal compressibility and the interaction between organic/inorganic ([CH3]4N-/MnX[image omitted] ) tetrahedra. The energy variation as a function of pressure is explained in terms of the isolated Td MnX[image omitted] in the respective crystals through crystal-field models. One salient feature is the reduction of the PL Stokes shift with pressure in both crystals as-1.0 and-1.7 nm/GPa for chloride and bromide, respectively. [ABSTRACT FROM AUTHOR]

This work investigates the Co2+(Td) electronic structure and photoluminescence (PL) in MgAl2O4:Co2+ and their variation with pressure by time-resolved spectroscopy. The absorption spectra and the pressure and temperature dependences of the emission band at 663 nm (τ=120 ns, at ambient conditions) can be explained on the basis of a configurational energy model, which provides an interpretation of the electronic structure and the excited-state phenomena. We show that there is an excited-state crossover [4T 1(P)↔2E (G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 and 100 K to a narrow-peak emission at 6 K. The pressure, instead of enhancing excited-state crossover phenomena, reduces PL and suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced non-radiative crossover relaxation to lower excited states: 2E (G)→4T 1(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation. [ABSTRACT FROM AUTHOR]

The structure of the Ga85.8In14.2 eutectic liquid alloy is investigated both under ambient conditions and at high pressure/high temperature using X‐ray absorption spectroscopy (XAS) and X‐ray diffraction (XRD) techniques. The local structure of the liquid alloy at ambient conditions is analyzed using double‐edge refinements of the XAS data. Solid–liquid phase transitions under high‐pressure and high‐temperature conditions are monitored by combined XAS and XRD measurements along several quasi‐isobaric heating runs, allowing to draw a melting line up to 10 GPa. The established melting line is found to be slightly below the one of pure gallium (Ga) and to follow its trend as expected from the eutectic nature of the compound. A series of Ga K‐edge X‐ray absorption fine structure (XAFS) spectra measured at different pressures indicates the absence of large structural modifications at local Ga sites in the liquid within the investigated pressure and temperature range. [ABSTRACT FROM AUTHOR]

The interplay between spin states and metallization in compressed CoCl2 is investigated by combining diffraction, resistivity and spectroscopy techniques under high-pressure conditions and ab-initio calculations. A pressure-induced metallization along with a Co2+ high-spin (S = 3/2) to low-spin (S = 1/2) crossover transition is observed at high pressure near 70 GPa. This metallization process, which is associated with the p-d charge-transfer band gap closure, maintains the localization of 3d electrons around Co2+, demonstrating that metallization and localized Co2+ -3d low-spin magnetism can coexist prior to the full 3d-electron delocalization (Mott-Hubbard d-d breakdown) at pressures greater than 180 GPa. [ABSTRACT FROM AUTHOR]

In order to disentangle the physical effects at the origin of transition metal K‐edge X‐ray magnetic circular dichroism (XMCD) in coordination polymers and quantify small structural distortions from the intensity of these signals, a systematic investigation of Prussian blue analogs as model compounds is being conducted. Here the effects of the temperature and of the external magnetic field are tackled; none of these external parameters modify the shape of the XMCD signal but they both critically modify its intensity. The optimized experimental conditions, as well as a reliable and robust normalization procedure, could thus be determined for the study of the intrinsic parameters. Through an extended discussion on measurements on other XMCD‐dedicated beamlines and for other coordination compounds, we finally provide new transition metal K‐edge XMCD users with useful information to initiate and successfully carry out their projects. [ABSTRACT FROM AUTHOR]

An A- and B-site ordered quadruple perovskite oxide LaCu3Co2Re2O12 was synthesized at 9 GPa and 1323 K. The compound possesses a Pn-3 space group, where both A and B sites are orderly occupied by different cations with a nearly 100% degree of order. Bond valence sum calculations and x-ray absorption spectroscopy confirm the charge distribution to be LaCu2+3Co2+2Re5.5+2O12. A ferrimagnetic phase transition is found to occur around 150 K due to the Cu2+(↑)-Co2+(↑)-Re5.5+(↓) spin coupling found by x-ray magnetic circular dichroism at Cu-, Co-, and Re-L2,3 edges. The magnetoresistance effects as well as the first-principle calculations indicate the half-metallic nature for LaCu3Co2Re2O12 with a wider energy gap at the up-spin channel and a conducting band at the down-spin channel. [ABSTRACT FROM AUTHOR]

We have developed a new internally heated diamond anvil cell (DAC) system for in situ high-pressure and high-temperature x-ray and optical experiments. We have adopted a self-heating W/Re gasket design allowing for both sample confinement and heating. This solution has been seldom used in the past but proved to be very efficient to reduce the size of the heating spot near the sample region, improving heating and cooling rates as compared to other resistive heating strategies. The system has been widely tested under high-temperature conditions by performing several thermal emission measurements. A robust relationship between electric power and average sample temperature inside the DAC has been established up to about 1500 K by a measurement campaign on different simple substances. A micro-Raman spectrometer was used for various in situ optical measurements and allowed us to map the temperature distribution of the sample. The distribution resulted to be uniform within the typical uncertainty of these measurements (5% at 1000 K). The high-temperature performances of the DAC were also verified in a series of XAS (x-ray absorption spectroscopy) experiments using both nano-polycrystalline and single-crystal diamond anvils. XAS measurements of germanium at 3.5 GPa were obtained in the 300 K–1300 K range, studying the melting transition and nucleation to the crystal phase. The achievable heating and cooling rates of the DAC were studied exploiting a XAS dispersive setup, collecting series of near-edge XAS spectra with sub-second time resolution. An original XAS-based dynamical temperature calibration procedure was developed and used to monitor the sample and diamond temperatures during the application of constant power cycles, indicating that heating and cooling rates in the 100 K/s range can be easily achieved using this device. [ABSTRACT FROM AUTHOR]

Polycrystalline Sr2Fe1+xRe1−xO6 samples have been synthesized and structurally characterized by X-ray powder diffraction, transmission electron microscopy and X-ray absorption spectroscopy. Resistivity strongly increases with x, but a large and negative magnetoresistance persists up to x = 0.33. This is discussed considering the charge delocalization in iron and rhenium t2g orbitals. [ABSTRACT FROM AUTHOR]

To investigate the interplay between electronic structure and itinerant magnetism, Ca1-xEuxCo2As2 solid solutions (x = 0, 0.1, 0.3, 0.4, 0.5, 0.6, 0.65, 0.7, 0.9, 1.0) were prepared by reactions between constituent elements in molten Bi. All of the samples crystallize in the ThCr2Si2 structure type. The crystal structure refinement revealed the formation of Co vacancies, the concentration of which decreases as the Eu content increases. The Eu site exhibits mixed valence in all samples. X-ray absorption near-edge structure spectroscopy revealed that the average Eu oxidation state decreases from +2.17 at 0 < x ≥ 0.6 to +2.14 at x ≤ 0.65. The same borderline behavior is observed in magnetic properties. The substitution of Eu for Ca causes the transition from the antiferromagnetic (AFM) ordering of Co moments in CaCo2As2 to ferromagnetic (FM) ordering of Co moments in Ca1-xEuxCo2As2 with 0.1 ≥ x ≥ 0.6. At higher Eu content, AFM ordering of Eu moments is observed, whereas the Co sublattice exhibits only paramagnetic behavior. Single-crystal neutron diffraction studies revealed that both Co and Eu sublattices order FM in Ca0.5Eu0.5Co2As2 with the magnetic moments aligned along the tetragonal c axis. In the AFM phases with x ≤ 0.65, only Eu moments are ordered in a helical spin structure defined by an incommensurate propagation vector k = [00q], with the moment lying in the ab plane. The changes in magnetic behavior are well-justified by the analysis of the electronic density of states and crystal orbital Hamilton population. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy (XAS) is presently a powerful and established tool to investigate solid and liquid matter at high pressure and high temperature (HP-HT). HP-HT XAS experiments rely on high pressure technology whose continuous development has extended the achievable range up to 100 GPa and more. In high pressure devices, high temperature conditions are typically obtained by using internal and external resistive heaters or by laser heating. We have recently developed a novel design for an internally heated diamond anvil cell (DAC) allowing XAS measurements under controlled high temperature conditions (tested up to about 1300 K). The sample in the new device can be rapidly heated or cooled (seconds or less) so the cell is suitable for studying melting/crystallization dynamics when coupled with a time-resolved XAS setup (second and sub-second ranges). Here we describe the internally heated DAC device which has been realized and tested in experiments on pure selenium at the energy dispersive ODE beamline of Synchrotron SOLEIL. We also present results obtained in XAS experiments of elemental Se using a large volume Paris-Edinburgh press, as an example of the relevance of structural studies of matter under extreme conditions. • Description of an internally heated diamond anvil cell (DAC) for time resolved XAS. • Successful DAC tests for XAS collection at high temperature on elemental Se. • Metalization in solid and liquid Se probed at high T and P by X-ray absorption. [ABSTRACT FROM AUTHOR]

X‐ray absorption spectroscopy is a powerful tool to probe in situ changes in the local structure of a material induced by pressure. In article no. 1800073 it was used by Kuzmin et al. to study the behavior of copper nitride (Cu3N) crystal lattice at high pressure (0–26.7 GPa). The analysis of the Cu K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) suggests that at all pressures the local atomic environment around copper atoms remains close to that in cubic Pm‐3m phase. At the same time, a reduction of the lattice parameter of Cu3N by about 2% was found upon increasing pressure up to 26.7 GPa. These results indicate that the transition to the metal state in Cu3N above 5 GPa, observed previously using pressure‐dependent electrical resistance and optical absorption measurements, is due to the band gap collapse with a decrease of the unit cell volume. [ABSTRACT FROM AUTHOR]

High‐pressure (0–26.7 GPa) Cu K‐edge X‐ray absorption spectroscopy is used to study possible structural modifications of anti‐perovskite‐type copper nitride (Cu3N) crystal lattice. The analysis of X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS), based on theoretical full‐multiple‐scattering and single‐scattering approaches, respectively, suggests that at all pressures the local atomic structure of Cu3N remains close to that in cubic Pm3¯m phase. Therefore, the transition to metal state above 5 GPa, observed previously using pressure‐dependent electrical resistance and optical absorption measurements, is explained by the band gap collapse due to a decrease of the unit cell volume. We found that the lattice parameter of Cu3N is reduced by ≈2% upon increasing pressure up to 26.7 GPa, and the structure is restored upon pressure release. Origin of pressure‐induced metallization in cubic Cu3N has been disclosed using in situ Cu K‐edge X‐ray absorption spectroscopy. The transition to metal state above 5 GPa is explained by the band gap collapse due to a decrease of the unit cell volume. The lattice parameter of Cu3N is reduced by ≈2% upon increasing pressure up to 26.7 GPa, and the structure is restored upon pressure release. The local atomic structure of Cu3N remains close to that in cubic Pm3¯m at all pressures. [ABSTRACT FROM AUTHOR]