Yann Le Godec

Study of cation order-disorder in MgAl2O4 spinel by in situ neutron diffraction up to 1600 K and 3.2 GPa

Abstract The temperature-dependence of the cation distribution in synthetic spinel (MgAl2O4) was investigated using in situ time-of-flight neutron powder diffraction at ISIS, the pulsed-neutron source at the Rutherford Appleton Laboratory. Neutron diffraction patterns of stoichiometric MgAl2O4 were collected on heating from room temperature to ∼1600 K at pressures of ∼2.6 GPa. The cation distribution was determined directly from site occupancies obtained by Rietveld refinement. The equilibrium non-convergent ordering was analyzed using the OʼNeill-Navrotsky (1983) thermodynamic model, which fits the observed behavior well over the temperature range of the measurements. Fitting the data between 790 and 1600 K yields α = 31(6) kJ/mol and β = -20(13) kJ/mol in the expression for the free energy of ordering. The high-pressure temperature-dependent behavior, as compared to equivalent ambient-pressure behavior, demonstrates that disordering occurs to a much greater extent in MgAl2O4 at high pressure and that pressure favors disordering toward the inverse structure.

A portable high-pressure stress cell based on the V7 Paris–Edinburgh apparatus

We describe a new device, based on a V7 Paris–Edinburgh press, for torsional testing of material at pressures up to 7 GPa (extendable to 15 GPa). Samples are deformed using a simple shear geometry between opposed anvils by rotating the lower anvil, via a rotational actuator, with respect to an upper, stationary, anvil. Use of conical anvil profiles greatly increases sample dimensions more than other high-pressure torsional apparatus did. Samples of polycrystalline Zr (2 mm thick, 3.5 mm diameter) have been sheared at strains exceeding γ ∼1.5 at constant strain rate and at pressures from 1.8 to 5 GPa, and textural development has been studied by electron microscopy. Use of amorphous-boron-epoxy gaskets means that nearly simple shear of samples can be routinely achieved. This apparatus allows study of the plastic and anelastic behaviour of materials under high pressure, and is particularly suited for performing in situ investigations using synchrotron or neutron radiation.

RECENT DEVELOPMENTS USING THE PARIS-EDINBURGH CELL FOR NEUTRON DIFFRACTION AT HIGH PRESSURE AND HIGH TEMPERATURE AND SOME APPLICATIONS

We describe a recently developed technique for high pressure high temperature neutron scattering up to 7 GPa and 2000 K with a novel radiographic method to determine the temperature under pressure. We demonstrate the technique by some recent results on hydrous minerals (namely brucite and ice VH) and on spinel MgAl2O4, where neutrons still have an unchallenged advantage over X-rays. Future scientific opportunities are outlined.

In situ studies of boron nitride crystallization from BN solutions in supercritical N–H fluid at high pressures and temperatures

The reaction of boron with products of the thermal decomposition of hydrazine and the crystallization of boron nitride from BN solutions in supercritical N–H fluid have been studies in situ at pressures up to 5.2 GPa and temperatures up to 1600 K using angle- and energy-dispersive X-ray diffraction with synchrotron radiation. Graphite-like boron nitride produced by the reaction between boron and N–H supercritical fluid dissolves in the latter to form associated solutions of different concentrations. On cooling, the disappearance of short-range order in this solution is observed, accompanied by the precipitation of solid phases (cBN or hBN and BN–NH3 intercalation compound depending on the pressure, temperature and concentration). Spontaneous crystallization of cubic boron nitride has been observed down to 1.9 ± 0.2 GPa, which is the lowest pressure of the cBN crystallization from a solution reported so far. The BN–NH3 hypothetical quasibinary section of the phase diagram of the B–N–H system at 4 GPa has been constructed.

A new high P-T cell for neutron diffraction up to 7 GPa and 2000 K with measurement of temperature by neutron radiography

Abstract This paper reports developments to enable neutron diffraction at simultaneous high temperatures and pressures using the Paris-Edinburgh cell. These include a new design of a cell assembly with internal heating. One of the novel features of our system is the use of neutron radiographic methods for measurement of temperature. Fully refinable neutron diffraction patterns obtained by time of flight technique with our apparatus are found to be of comparable quality to previous high-pressure studies at ambient temperatures. In this paper we describe the procedures for the generation and measurement of pressure and temperature and illustrate the quality of the data which can be obtained. The present system may be used on a routine basis for experiments up to 7 GPa and temperature approaching 2000 K. Current attempts are discussed for extending these measurements to a wider domain of pressures and temperatures.

Boron phosphide under pressure: In situ study by Raman scattering and X-ray diffraction

Cubic boron phosphide, BP, has been studied in situ by X-ray diffraction and Raman scattering up to 55GPa at 300K in a diamond anvil cell. The bulk modulus of B0¼174(2) GPa has been established, which is in excellent agreement with our ab initio calculations. The data on Raman shift as a function of pressure, combined with equation-of-state (EOS) data, allowed us to estimate the Gr€uneisen parameters of the TO and LO modes of zinc-blende structure, cG TO¼1.26 and cG LO¼1.13, just like in the case of other AIIIBV diamond-like phases, for which cG TO>cG LOffi1. We also established that the pressure dependence of the effective electro-optical constant a is responsible for a strong change in relative intensities of the TO and LO modes from ITO/ILO 0.25 at 0.1MPa to ITO/ILO 2.5 at 45GPa, for which we also find excellent agreement between experiment and theory

X-ray absorption study of Cu, Zn SOD under high pressure

Abstract We have investigated Cu, Zn Superoxide Dismutase (Cu, Zn SOD) metal sites at high pressure using X-ray absorption. XAS (X-ray Absorption Spectroscopy) gives information on local structure and it is particularly suited to metal site investigation. To the best of our knowledge, this is the first time that protein conformational states have been investigated using the high pressure XAS technique. Cu, Zn SOD catalyses the dismutation of toxic oxygen radicals produced in cells; this reaction occurs at the copper metal site. Structural changes around the copper, induced by pressure, can be directly related to protein substates. Their characterisation is thus important in the understanding of protein activity. The high-pressure device was a Paris-Edinburgh large volume cell. Experiments were performed on lyophilised Cu, Zn SOD between 0 and 48 kbar at the copper and zinc K-edges. The two metal local atomic environments have a different behaviour as pressure increases: copper exhibits a more flexible environment; on the contrary, zinc shows small structural modifications. We have identified a state, formed between 3 and 8 kbar, which is stable up to 48 kbar.

Orthorhombic boron oxide under pressure: In situ study by X-ray diffraction and Raman scattering

High-pressure phase of boron oxide, orthorhombic β-B2O3, has been studied in situ by synchrotron X-ray diffraction to 22 GPa and Raman scattering to 46 GPa at room temperature. The bulk modulus of β-B2O3 has been found to be 169(3) GPa that is in good agreement with our ab initio calculations. Raman and IR spectra of β-B2O3 have been measured at ambient pressure; all experimentally observed bands have been attributed to the theoretically calculated ones, and the mode assignment has been performed. Based on the data on Raman shift as a function of pressure, combined with equation-of-state data, the Gruneisen parameters of all experimentally observed Raman bands have been calculated. β-B2O3 enriched by 10B isotope has been synthesized, and the effect of boron isotopic substitution on Raman spectra has been studied.

Optical phonon modes in rhombohedral boron monosulfide under high pressure

Raman spectra of rhombohedral boron monosulfide (r-BS) were measured under pressures up to 34 GPa at room temperature. No pressure-induced structural phase transition was observed, while strong pressure shift of Raman bands towards higher wavenumbers has been revealed. IR spectroscopy as a complementary technique has been used in order to completely describe the phonon modes of r-BS. All experimentally observed bands have been compared with theoretically calculated ones and modes assignment has been performed. r-BS enriched by 10B isotope was synthesized, and the effect of boron isotopic substitution on Raman spectra was observed and analyzed.Raman spectra of rhombohedral boron monosulfide (r-BS) were measured under pressures up to 34 GPa at room temperature. No pressure-induced structural phase transition was observed, while strong pressure shift of Raman bands towards higher wavenumbers has been revealed. IR spectroscopy as a complementary technique has been used in order to completely describe the phonon modes of r-BS. All experimentally observed bands have been compared with theoretically calculated ones and modes assignment has been performed. r-BS enriched by 10B isotope was synthesized, and the effect of boron isotopic substitution on Raman spectra was observed and analyzed.

Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways.

Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and ∼900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures ∼1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, (29)Si NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.