On the High-Pressure Phase Transition in GaPO4

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO4 at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a=7.306 A, b=5.887 A and c=5.124 A.Comment: 7 pages, 2 figures, 1 tabl

XMCD under pressure at the Fe K edge on the energy dispersive beamline of the ESRF

The present paper demonstrates the feasibility of X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) at high pressure at the Fe-K edge on the ID24 energy dispersive beamline of the ESRF. In 3d transition metals, performing experiments at the hard X-ray K-edge rather than at the magnetically interesting soft X-ray L-edges represents, the only way to access the high pressure regime obtainable with Diamond Anvil Cells. The simultaneous availability of a local structure (XAS) and of a magnetic (XMCD) probe on the sample in identical thermodynamical conditions is essential to study correlations between local structural and magnetic properties. We briefly summarize the state of the art theoretical understanding of K-edge XMCD data, then illustrate the setup of beamline ID24 for high pressure XMCD experiments and underline the conditions required to perform measurements at the K-edges of 3d transition metals. Finally, we present two examples of recent high pressure results at the Fe-K edge in pure Fe and Fe3O4 powder

Thermodynamics by synchrotron X-ray diffraction: phase relationships and crystal structure of L-tyrosine ethyl ester form III

In the case of small organic molecules, phase behaviour, which is important for pharmaceutical applications, is often only studied as a function of temperature. However, for a full thermodynamic description, not only the temperature but also the pressure should be taken into account, because pressure and temperature are the two characteristic variables for the Gibbs energy. The commercial form of L-tyrosine ethyl ester has been studied by synchrotron X-ray diffraction while subjected to different pressures and temperatures. At room temperature, it turns into a new previously unknown form around 0.45 GPa. The structure has been solved with an orthorhombic unit cell, space group P2(1)2(1)2(1), with parameters a = 12.655(4)angstrom, b = 16.057(4)angstrom, c = 5.2046(12)angstrom, and V = 1057.6(5)angstrom(3) at T = 323 K and P = 0.58 GPa. The enthalpy of the transition from the commercial form to the new form could be estimated from the slope of the transition obtained from the synchrotron diffraction data. In addition, the topological pressure-temperature phase diagram has been constructed involving the two solid phases, the liquid and the vapour phase. The solid phases are enantiotropic under low pressure, but the system becomes monotropic at high pressure with the new solid phase being the only stable one.Postprint (author's final draft

Melting properties of pyrolite: implication for chemical segregation in the primitive Earth’s mantle.

International audienc

Thermodynamics by synchrotron X-ray diffraction: phase relationships and crystal structure of L-tyrosine ethyl ester form III

In the case of small organic molecules, phase behaviour, which is important for pharmaceutical applications, is often only studied as a function of temperature. However, for a full thermodynamic description, not only the temperature but also the pressure should be taken into account, because pressure and temperature are the two characteristic variables for the Gibbs energy. The commercial form of L-tyrosine ethyl ester has been studied by synchrotron X-ray diffraction while subjected to different pressures and temperatures. At room temperature, it turns into a new previously unknown form around 0.45 GPa. The structure has been solved with an orthorhombic unit cell, space group P2(1)2(1)2(1), with parameters a = 12.655(4)angstrom, b = 16.057(4)angstrom, c = 5.2046(12)angstrom, and V = 1057.6(5)angstrom(3) at T = 323 K and P = 0.58 GPa. The enthalpy of the transition from the commercial form to the new form could be estimated from the slope of the transition obtained from the synchrotron diffraction data. In addition, the topological pressure-temperature phase diagram has been constructed involving the two solid phases, the liquid and the vapour phase. The solid phases are enantiotropic under low pressure, but the system becomes monotropic at high pressure with the new solid phase being the only stable one

Compressibility of structural modulation waves in the chain compounds BaCoX2O7 ( X = As, P): a powder study

International audienceBaCoX2O7 ( X = As, P) are built on magnetic 1D units in which strong aperiodic undulations originate from incommensurate structural modulations with large atomic displacive amplitudes perpendicular to the chain directions, resulting in very unique multiferroic properties. High-pressure structural and vibrational properties of both compounds have been investigated by synchrotron X-ray powder diffraction and Raman spectroscopy at room temperature and combined with density functional calculations. A structural phase transition is observed at 1.8 GPa and 6.8 GPa in BaCoAs 2 O 7 and BaCoP2O 7 , respectively. Sharp jumps are observed in their unit-cell volumes and in Raman mode frequencies, thus confirming the first-order nature of their phase transition. These transitions involve the disappearance of the modulation from the ambient-pressure polymorph with clear spectroscopic fingerprints, such as reduction of the number of Raman modes and change of shape on some peaks. The relation between the evolution of the Raman modes along with the structure are presented and supported by density functional theory structural relaxations

Disorder-order phase transition at high pressure in ammonium fluoride

Solid NH4F displays intriguing parallels with ice despite its apparently ionic character. Here we investigate its phase diagram in low-temperature and high-pressure regions using Raman spectroscopy, X-ray diffraction and ab initio structure search calculations. We focus on the high-pressure cubic phase which resembles that found in ice under pressure and is also the ambient pressure phase of other ammonium halides. We detect a disorder-order transition above 10 GPa, recalling those found both in other ammonium halides and in ice. The transition reveals itself in the pressure dependence of several Raman modes as well as through the progressive splitting of lattice and bending modes of the cubic phase at low temperatures. An in-depth analysis of the Raman modes and their evolution is made

Charge Order and Suppression of Superconductivity in HgBa2CuO4+d at High Pressures

International audienceNew insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20 GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to similar to 10 GPa that gradually passes into an insulating state, typical of charge ordering, which totally suppresses superconductivity above 13 GPa. The changes in resistivity are accompanied by the apparition of sharp Bragg peaks in the X-ray diffraction patterns, indicating that the charge ordering is accompanied by a 3D oxygen ordering. Considering that pressure induces a charge transfer of about 0.02 at 10 GPa, our results are the first observation of charge order competing with superconductivity developed in the overdoped region of the phase diagram of a Hg-based cuprate

Materiaux sous hautes pressions : etude in situ des proprietes electroniques et structurales par spectroscopie d'absorption x

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