F. Baudelet

Dynamics of the Magnetic and Structural alpha-epsilon Phase Transition in Iron

We have studied the high-pressure iron bcc to hcp phase transition by simultaneous x-ray magnetic circular dichroism and x-ray absorption spectroscopy with an x-ray energy dispersive spectrometer. The combination of the two techniques allows us to obtain simultaneously information on both the structure and the magnetic state of iron under pressure. The magnetic and structural transitions simultaneously observed are sharp. Both are of first order in agreement with the theoretical prediction. The pressure domain of the transition observed (2.4+/-0.2 GPa) is narrower than that usually cited in the literature (8 GPa). Our data indicate that the magnetic transition slightly precedes the structural one, suggesting that the origin of the instability of the bcc phase in iron with increasing pressure is to be attributed to the effect of pressure on magnetism as predicted by spin-polarized full-potential total energy calculations.

High-pressure phase transformation of nanometric ZnSb prepared by mechanical alloying

Nanometric orthorhombic ZnSb was prepared by mechanical alloying and its structural stability was studied as a function of pressure. The changes were followed by Raman and x-ray absorption spectroscopy. Between 11.0 and 14.6 GPa, we observed an irreversible phase transformation from the orthorhombic phase to a hexagonal hP1 phase. A similar transformation had been previously observed at 7 GPa for bulk ZnSb. The difference in the phase transformation pressures is attributed to an interfacial component of the nanometric structure.

High pressure x-ray absorption spectroscopy at lower energy in the dispersive mode: application to Ce and FePO4

X-ray absorption spectroscopy (XAS) using a diamond anvil cell has been extended to lower energies (below 6 keV) thanks to smaller diamond anvils and to a new acquisition set-up on the energy dispersive x-ray absorption beamline of LURE. The valence change of Ce with pressure is followed at the LIII edge of cerium up to 8 GPa together with the bond length variation during the same experiment. The pressure induced coordination change around the Fe atom in FePO4 is determined at the Fe K edge through the pre-edge feature behaviour, the XANES modification and the Fe–O bond length increase.

Short-range order of compressed amorphous GeSe2

The structure of amorphous GeSe2 (a-GeSe2) has been studied by means of a combination of two-edges X-ray absorption spectroscopy (XAS) and angle-dispersive X-ray diffraction under pressures up to about 30u2009GPa. Multiple-edge XAS data-analysis of a-GeSe2 at ambient conditions allowed us to reconstruct and compare the first-neighbor distribution function with previous results obtained by neutron diffraction with isotopic substitution. GeSe2 is found to remain amorphous up to the highest pressures attained, and a reversible 1.5u2009eV red-shift of the Ge K-edge energy indicating metallization, occurs between 10u2009GPa and 15u2009GPa. Two compression stages are identified by XAS structure refinement. First, a decrease of the first-neighbor distances up to about 10u2009GPa, in the same pressure region of a previously observed breakdown of the intermediate-range order. Second, an increase of the Ge-Se distances, bond disorder, and of the coordination number. This stage is related to a reversible non-isostructural transition involving a gradual conversion from tetra- to octa-hedral geometry which is not yet fully completed at 30u2009GPa.

Two recent developments in XMCD.

This paper reports on two new technical developments concerning sample environments for X-ray magnetic circular dichroism (XMCD). The first measurements under high pressures of up to 30 GPa are described. The difficulties of combining the techniques of high pressure and XMCD are commented on. The second development involves the use of a fast-switching magnetic field. A new superconducting device is used to perform XMCD measurements on paramagnetic compounds in magnetic fields of up to 6 T. The small amplitude of the XMCD signal imposes, for a given signal-to-noise ratio, a noise less than a few 10(-5). The signal-to-noise ratio is improved by the use of a series of acquisitions, switching the magnetic field between each acquisition. A very fast switching mechanism has been built based on mechanical rotation of a superconducting coil, with the sample kept in place inside the coil. The XMCD signals at the L(II,III)-edges of paramagnetic rare-earth compounds have been measured at 4.5 K in fields of up to 6 T with a switching time of 11 s.

Amorphous germanium under high-pressure conditions

In this work, we address the question of pressure-induced phase transitions in amorphous germanium. We discuss both the pressurization and depressurization cycles on an amorphous film, showing the appearance of various stable and metastable (LDA–HDA) phases by Raman scattering and X-ray absorption spectroscopy. In particular, we report evidence of recrystallization in the metastable ST12 phase driven by the density of voids of the film.

Pressure Dependence of Wurtzite ZnO Structure

From simultaneous X-ray diffraction and EXAFS experiments, structural properties in the Wurtzite phase of ZnO have been measured up to the transition pressure (9 GPa). These results are compared with first-principles calculations and discussed in terms of wurtzite mechanical stability at high pressure.

Magnetic and crystallographic characterization of Pt3MnxCr1−x by XMCD and x-ray diffraction

We have performed XMCD and diffraction measurements on the Pt(3)Mn(x)Cr(1-x) alloy, which show that the magnetization of Pt is independently influenced by the Mn or Cr 3d orbital. We find that the magnetic moment on Pt, and its decomposition into spin and orbital components, is uniquely determined by the relative number of Mn and Cr neighbors. We then investigate the effect of pressure on the magnetization of Pt in the Pt(3)Mn(0.5)Cr(0.5) alloy. Our high pressure data enable us to conclude that at 14xa0GPa the spin and orbital polarization of the Ptxa05d band are augmented by about 70%, with no interaction between them.