Proton configurations in the hydrogen bonds of KH2PO4 as seen by resonant x-ray diffraction

KH2PO4 (KDP) belongs to the class of hydrogen-bonded ferroelectrics, whose paraelectric to ferroelectric phase transition is driven by the ordering of the protons in the hydrogen bonds. We demonstrate that forbidden reflections of KDP, when measured at an x-ray absorption edge, are highly sensitive to the asymmetry of proton configurations. The change of average symmetry caused by the "freezing" of the protons during the phase transition is clearly evidenced. In the paraelectric phase, we identify in the resonant spectra of the forbidden reflections a contribution related to the transient proton configurations in the hydrogen bonds, which violates the high average symmetry of the sites of the resonant atoms. The analysis of the temperature dependence reveals a change of relative probabilities of the different proton configurations. They follow the Arrhenius law, and the activation energies of polar and Slater configurations are 18.6 and 7.3 meV, respectively

Resonant x-ray scattering study of charge superstructures in layered L a2-x C axCo O4±d (0.4=x=0.7) and L a1.5 S r0.5Co O4 compounds

We propose a model for the Co charge disproportionation in La2-xCaxCoO4±d (0.4=x=0.7) and La1.5Sr0.5CoO4 layered cobaltites based on the photon energy, azimuthal angle, and polarization dependence of the scattered intensity of selected reflections in a resonant x-ray scattering experiment. Tetragonal superlattice (h/2, h/2, l)-type reflections were detected at room temperature for all the samples independently from the Ca (or Sr) doping rate in agreement with a checkerboard ordering of two different Co sites. The corresponding average charge disproportion is accounted for by a semiempirical model from which the imaginary part of the resonant atomic scattering factors of each of the two Co atoms is obtained resulting in about 0.5±0.1 electron. Moreover, no forbidden (h/4, h/4, l)-type reflections were observed at room temperature indicating the lack of any local anisotropy ordering in contrast with the behavior found in the related layered manganites. Finally, we found the pattern of small distortions (~0.05Å) around the two Co sites compatible with the resonant x-ray scattering results. The symmetry of this displacement pattern is consistent with the A2mm (or Ammm) orthorhombic structure of the ordered phase and the structural transition is accounted for by the condensation of two soft modes - X1+(B2u) and X1+(A1) - acting on the oxygen atoms

Compression of gold sub-micron crystallites: Method and experiments

Understanding and characterizing the mechanical response of individual nanostructure is of great importance for both fundamental prospects and device reliability. Higher flow stress with decreasing sample size is observed together with jerky flow. Compression of pristine submicron gold crystallites yield at very large stress in a stochastic manner, followed by large displacement bursts reaching up to 50% of the initial height [1,2]. In this work, by collecting a large set of measurements, we investigate the small and large strain behavior of crystallites loaded in compression. Large arrays of [111] oriented gold crystallites are prepared by solid state dewetting of initial cylinders of different volumes on sapphire substrates. Dedicated flat punch compression in-situ a FEG-SEM (figure 1a) has been carried out in load controlled mode [3]. Microstructure of defects is investigated using synchrotron radiation by nanoscale 3D imaging (Bragg Coherent X-ray Diffraction Imaging) [4] and Atomic Force Microscopy observations. The analysis of the plastic instability and its amount of deformation is carried out taking into account the inertial effect of the instrument, using a 1D dynamic model and Finite Element Method calculations. Simulations are made with different estimates of the shape of each individual crystallite, from an ideal cylinder of equivalent volume to the one based on SEM or AFM observations. We show that prior to the displacement burst, plastic events take place and that the sudden displacement does not necessarily relates to the onset of dislocation nucleation (figure 1b). Moreover, using the collection of measurements, we show that a unique stress-strain response can be obtained which can be used as a lower bound estimate of the mechanical response in compression of the crystallites. Please click Additional Files below to see the full abstract

Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering

We report resonant X-ray scattering measurements on the orbitally-degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K-edge. The empirically extracted K-edge shift between the crystallographically-distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ~1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.Comment: 5 pages, 4 figure

An X-ray study of the Dzyaloshinskii-Moriya interaction in the weak ferromagnet FeBO3

International audienceWe report on the axis, magnitude and direction of the Dzyaloshinskii-Moriya (DM) interaction in the weak ferromagnet FeBO3. The latter relies on the determination of the phase of the magnetic x-ray scattering amplitude. We outline a new technique based on interference with forbidden quadrupole resonant scattering to obtain this phase information

Phase transition of KDP observed by Resonant X-ray Diffraction at forbidden reflections

International audienceWe report observations of space-group-forbidden Bragg reflections in Potassium (KH2PO4), at the potassium K edge. We find clear evidence for a transition from one class of space-group-forbidden reflections, where scattering is ruled out by the electric dipole approximation, to a second class, in the ferroelectric phase, where scattering can proceed due to resonant anisotropy within the dipole approximation. The change of symmetry is clearly evidenced by the sudden change of intensity and energy spectrum of the forbidden reflections

Antiferromagnetic order and the structural order-disorder transition in the Cd6Ho quasicrystal approximant

International audienceIt has generally been accepted that the orientational ordering of the Cd4 tetrahedron within the Cd6 R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd6Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T S = 178 K for samples that have 'aged' at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L 3-edge we have elucidated the nature of the antiferromagnetic ordering below T N = 8.5 K in Cd6Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd6Tb

Magnetically induced femtoscale strain modulations in HoMn2O5

International audienceX-ray scattering was used to investigate the magnetically induced ionic displacements in the low-temperature commensurate ferroelectric/antiferromagnetic phase of the multiferroic HoMn2O5. The structural modulation signal appearing at twice the magnetic wave vector km=(1/201/4) has been used, combined with symmetry analysis, to determine a model for the ionic displacements up to a precision of 10-3Å. The symmetry-breaking operations that are associated to the active irreducible representation have been experimentally determined from the analysis of the modulation modes. They reveal a lowering of symmetry to the polar point group m2m. Calculations based on the determined model show that the magnetic structure along the c direction is stabilized via nearest-neighbor and next-nearest-neighbor interactions through the rare-earth layer. We also show that by knowing both the magnetic and the magnetically induced strain waves patterns, it is possible to remove any phase ambiguity between the crystal and magnetic structures