We report on a Ni L2,3 edges x-ray absorption spectroscopy (XAS) study in RNiO3 perovskites. These compounds exhibit a metal to insulator (M I) transition as temperature decreases. The L3 edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the M I transition in RNiO3 perovskites in terms of modifications in the Ni 3+ crystal field splitting that induces a spin transition from an essentially low-spin (LS) to a mixed-spin state.PACS numbers: 61.10. Ht, 71.30.+h, 75.10.Dg, 75.25.+z Keywords: x-ray absorption spectroscopy, XAS, metal-insulator transition, charge transfer multiplet theory Rare-earth nickel perovskites (RNiO 3 , R=rare earth) present a sharp well-defined metal to insulator (M I) transition as temperature decreases 1 . The transition temperature, T MI , increases with reducing the R ion size, which determines the degree of distortion of the structure 2 . It was proposed that the gap opening would be due to a smaller Ni-O-Ni superexchange angle leading to a reduction of the bandwidth 3 . However, nonnegligible electron-phonon interactions 4 and a shift in T MI with oxygen isotope substitution 5 evidenced the importance of modifications in Ni-O interatomic distances, suggesting a phonon assisted mechanism for conduction. As temperature decreases, these nickelates undergo a magnetic transition to an unusual antiferromagnetic order 6,7,8 . The magnetic arrangement for the lighter R compounds (R=Pr, Nd, Sm, Eu) was refined with a single Ni moment (0.9µ B ) and required non-equivalent couplings among Ni ions to stabilize the structure 6,7 . This is a quite unusual situation in an orthorhombic crystallographic structure whose Ni sites are all equivalent 2 . For the heavier R compounds, Alonso et al.8,9 established a monoclinic distortion in the crystallographic structure leading to two different Ni sites with longer and shorter Ni-O distances alternating along the three axis. The antiferromagnetic structure was explained by a charge ordering defined among the different Ni sites, each one with different magnetic moments (1.4 and 0. low-spin configuration, because the amount of charge transfer for parallel spin almost equals that for antiparallel spin 13 . However, the spectral shape in RNiO 3 compounds is very sensitive to the transition from metallic to insulating states 15 and a complete description of the spin degree of freedom remains to be given. As in recent outcomes on Co 3+ oxides 16,17 , where unconventional spin states exist due to the competition between crystal field splitting and effective 3d exchange interaction, assignments made so far about Ni 3+ in RNiO 3 compounds have to be reexamined.We report here Ni L-edge absorption measurements, which probes directly the available Ni3d states, together with charge transfer multiplet calculations. We establish the importance of the cry...
The short-range organization around Ni atoms in orthorhombic RNiO 3 ͑R = Pr, Nd, Eu͒ perovskites has been studied over a wide temperature range by Ni K-edge x-ray-absorption spectroscopy. Our results demonstrate that two different Ni sites, with different average Ni-O bond lengths, coexist in those orthorhombic compounds and that important modifications in the Ni nearest-neighbors environment take place across the metal-insulator transition. We report evidences for the existence of short-range charge order in the insulating state, as found in the monoclinic compounds. Moreover, our results suggest that the two different Ni sites coexist even in the metallic state. The coexistence of two different Ni sites, independently on the R ion, provides a common ground to describe these compounds and sheds interesting light in the understanding of the phonon-assisted conduction mechanism and unusual antiferromagnetism present in all RNiO 3 compounds.
International audienceTetrathiafulvalene-chloranil (TTF-CA) was synthesized by two methods, liquid assisted grinding (LAG) and vapor digestion (VD), which largely reduce the use of reaction solvents. The effects of the small quantities of LAG solvent and solvent vapors in VD toward the formation of a particular TTF-CA product polymorph were studied from both tetrathiafulvalene forms (orange and brown) as reactants. It was concluded that a high solvent polarity index favors the formation of the ionic black polymorph of TTF-CA vs the quasi-neutral green form, whereas the crystal structure and crystal habit of the orange tetrathiafulvalene polymorph also favors the formation of the black TTF-CA. The crystal structure of the black TTF-CA was determined from synchrotron X-ray powder diffraction (XRPD), and it consists of dimerized TTF+center dot and CA(-center dot) radical ions, in agreement with room temperature magnetic susceptibility measurements indicating the material is diamagnetic. FT-IR showed that the compound is a semiconductor with a small. band gap of similar to 0.198 eV and it remains ionic at low temperatures. The latter was confirmed by XRPD showing the black TTF-CA does not undergo a phase transition in the range 298-20 K. Band structure calculations are in good agreement with the measured band gap
An energy-dispersive X-ray absorption spectroscopy beamline mainly dedicated to X-ray magnetic circular dichroism (XMCD) and material science under extreme conditions has been implemented in a bending-magnet port at the Brazilian Synchrotron Light Laboratory. Here the beamline technical characteristics are described, including the most important aspects of the mechanics, optical elements and detection set-up. The beamline performance is then illustrated through two case studies on strongly correlated transition metal oxides: an XMCD insight into the modifications of the magnetic properties of Cr-doped manganites and the structural deformation in nickel perovskites under high applied pressure.
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