XRD and XAS structural study of CuAlO2 under high pressure

We present the results of x-ray diffraction and x-ray absorption spectroscopy experiments in CuAlO(2) under high pressure. We discuss the polarization dependence of the x-ray absorption near-edge structure at the Cu K-edge. XRD under high pressure evidences anisotropic compression, the a-axis being more compressible than the c-axis. EXAFS yields the copper-oxygen bond length, from which the only internal parameter of the delafossite structure is deduced. The combination of anisotropic compression and the internal parameter decrease results in a regularization of the AlO(6) octahedra. The anisotropic compression is related to the chemical trends observed in the lattice parameters when Al is substituted by other trivalent cations. Both experiments evidence the existence of an irreversible phase transition that clearly manifests at 35 ± 2 GPa. The structure of the high-pressure polymorph could not be determined, but it implies a change of the Cu environment, which remains anisotropic. Precursor effects are observed from the lowest pressures, which are possibly related to crystal breaking at a submicroscopic scale with partial reorientation of the crystallites.

CO2-helium and CO2-neon mixtures at high pressures

The properties of mixtures of carbon dioxide with helium or neon have been investigated as a function of CO(2) concentration and pressure up to 30 GPa at room temperature. The binary phase diagrams of these mixtures are determined over the full range of CO(2) concentrations using visual observations and Raman scattering measurements. Both diagrams are of eutectic type, with a fluid-fluid miscibility gap for CO(2) concentrations in the range [5, 75] mol. % for He and [8, 55] mol. % for Ne, and a complete separation between the two components in the solid phase. The absence of alloys or stoichiometric compounds for these two binary systems is consistent with the Hume-Rothery rules of hard sphere mixtures. The Raman spectra and x-ray diffraction patterns of solid CO(2) embedded in He or Ne for various initial concentrations have been measured up to 30 GPa and 12 GPa, respectively. The frequencies of the Raman modes and the volume of solid phase I are identical, within error bars, to those reported for 100% CO(2) samples, thus confirming the total immiscibility of CO(2) with He and Ne in the solid phase. These results demonstrate the possibility to perform high-pressure experiments on solid CO(2) under (quasi-)hydrostatic conditions using He or Ne as pressure transmitting medium.

Iron under pressure: "Kohn tweezers" and remnant magnetism

In this work we investigate the magnetic and structural properties of bulk Fe and Fe nanoparticles under pressure with x-ray absorption and emission spectroscopies providing answers to two fundamental questions: (a) the chicken-or-egg problem for the magnetic and structural transitions and (b) magnetism in the high pressure hcp phase. The two transitions, inextricably linked in the bulk, are clearly decoupled in the nanoparticles, with the magnetic collapse preceding the structural transition. Ultrafast x-ray emission spectroscopy detects remnant magnetism, probably antiferromagnetic fluctuations, up to pressures of about 40 GPa in the hcp phase. This could be of direct relevance to the superconductivity in ϵ-Fe [K. Shimizu et al., Nature (London) 412, 316 (2001)] through the existence of a quantum critical point and associated magnetic fluctuations.

Liquid mercury sound velocity measurements under high pressure and high temperature by picosecond acoustics in a diamond anvils cell

Recent improvements to measure ultrasonic sound velocities of liquids under extreme conditions are described. Principle and feasibility of picosecond acoustics in liquids embedded in a diamond anvils cell are given. To illustrate the capability of these advances in the sound velocity measurement technique, original high pressure and high temperature results on the sound velocity of liquid mercury up to 5 GPa and 575 K are given. This high pressure technique will certainly be useful in several fundamental and applied problems in physics and many other fields such as geophysics, nonlinear acoustics, underwater sound, petrology or physical acoustics.

Experimental and Theoretical Investigation on the Relative Stability of the PdS2- and Pyrite-Type Structures of PdSe2

Under ambient condition PdSe2 has the PdS2-type structure. The crystal structure of PdSe2 under pressure (up to 30 GPa) was investigated at room temperature by X-ray diffraction in an energy-dispersive configuration using a diamond anvil cell with a mixture of water/ethanol/methanol as a pressure transmitting medium. A reversible structural transition from the PdS2-type to the pyrite-type structure occurs around 10 GPa, and the applied pressure reduces the spacing between adjacent 2/proportional to [PdSe2] layers of the PdS2-type structure to form the three-dimensional lattice of the pyrite-type structure. First principles and extended Hückel electronic band structure calculations were carried out to confirm the observed pressure-induced structural changes. We also examined why the isoelectronic analogues NiSe2 and PtSe2 adopt structures different from the PdS2-type structure on the basis of qualitative electronic structure considerations.

The structure of a pyoverdine produced by a Pseudomonas tolaasii-like isolate

Cultures of Agaricus bisporus, the most extensively cultivated mushroom, can be infected severely by Pseudomonas tolaasii. This pathogen is characterized by the so-called white line reaction, a precipitate formed on agar plates between its colonies and those of P. reactans, both belonging to the collective species P. fluorescens. A recent study has shown that a group of P. tolaasii isolates can be subdivided into two groups or 'siderovars', based on the pyoverdines they produce (Munsch et al. 2000). One group of strains is characterized by the pyoverdine described by Demange et al. (1990). A representative of the second group (strain Ps3a) was found to produce the same pyoverdine as a strain which had been classified before as P. aureofaciens. However, based mainly on 16S rRNA gene sequence comparisons and REP-PCR generated fingerprints, the two strains are not identical. They are also distinguishable from the P. tolaasii type strain.

Application of siderotyping for characterization of Pseudomonas tolaasii and "Pseudomonas reactans" isolates associated with brown blotch disease of cultivated mushrooms

Pyoverdine isoelectric focusing analysis and pyoverdine-mediated iron uptake were used as siderotyping methods to analyze a collection of 57 northern and central European isolates of P. tolaasii and "P. reactans." The bacteria, isolated from cultivated Agaricus bisporus or Pleurotus ostreatus mushroom sporophores presenting brown blotch disease symptoms, were identified according to the white line test (W. C. Wong and T. F. Preece, J. Appl. Bacteriol. 47:401-407, 1979) and their pathogenicity towards A. bisporus and were grouped into siderovars according to the type of pyoverdine they produced. Seventeen P. tolaasii isolates were recognized, which divided into two siderovars, with the first one containing reference strains and isolates of various geographical origins while the second one contained Finnish isolates exclusively. The 40 "P. reactans" isolates divided into eight siderovars. Pyoverdine isoelectric focusing profiles and cross-uptake studies demonstrated an identity for some "P. reactans" isolates, with reference strains belonging to the P. fluorescens biovars II, III, or V. Thus, the easy and rapid methods of siderotyping proved to be reliable by supporting and strengthening previous taxonomical data. Moreover, two potentially novel pyoverdines characterizing one P. tolaasii siderovar and one "P. reactans" siderovar were found.