Florent Occelli

Synthesis of lithium polyhydrides above 130 GPa at 300 K

Significance High hydrides with unusual stoichiometries have been predicted to become energetically favored in various hydrides of alkali and alkali earth metals under pressure. This paper reports on synchrotron infrared spectroscopic measurements on lithium hydride (LiH) compressed in a diamond anvil cell up to 215 GPa, showing that insulating lithium polyhydrides containing H2 units are synthesized above 130 GPa at 300 K. The observed vibron frequencies are in good agreement with the predictions for LiH2 and LiH6. The prediction of novel lithium hydrides with nontraditional stoichiometries at high pressure has been seminal for highlighting a promising line of research on hydrogen-dense materials. Here, we report the evidences of the disproportionation of LiH above 130 GPa to form lithium hydrides containing H2 units. Measurements have been performed using the nonperturbing technique of synchrotron infrared absorption. The observed vibron frequencies match the predictions for LiH2 and LiH6. These polyhydrides remain insulating up to 215 GPa. A disproportionation mechanism based on the diffusion of lithium into the diamond anvil and a stratification of the sample into LiH6/LiH2/LiH layers is proposed. Polyhydrides containing an H2 sublattice do exist and could be ubiquitously stable at high pressure.

Equation of state of rhenium and application for ultra high pressure calibration

The isothermal equation of state of rhenium has been measured by powder X-ray diffraction experiments up to 144 GPa at room temperature in a diamond anvil cell. A helium pressure transmitting medium was used to minimize the non-hydrostatic stress on the sample. The fit of pressure-volume data yields a bulk modulus K0 = 352.6 GPa and a pressure derivative of the bulk modulus K′0=4.56. This equation of state differs significantly from a recent determination [Dubrovinsky et al., Nat. Commun. 3, 1163 (2012)], giving here a lower pressure at a given volume. The possibility of using rhenium gasket X-ray diffraction signal, with the present equation of state, to evaluate multi-Mbar pressures in the chamber of diamond anvil cells is discussed.

Probing local and electronic structure in Warm Dense Matter: single pulse synchrotron x-ray absorption spectroscopy on shocked Fe

Understanding Warm Dense Matter (WDM), the state of planetary interiors, is a new frontier in scientific research. There exists very little experimental data probing WDM states at the atomic level to test current models and those performed up to now are limited in quality. Here, we report a proof-of-principle experiment that makes microscopic investigations of materials under dynamic compression easily accessible to users and with data quality close to that achievable at ambient. Using a single 100 ps synchrotron x-ray pulse, we have measured, by K-edge absorption spectroscopy, ns-lived equilibrium states of WDM Fe. Structural and electronic changes in Fe are clearly observed for the first time at such extreme conditions. The amplitude of the EXAFS oscillations persists up to 500 GPa and 17000 K, suggesting an enduring local order. Moreover, a discrepancy exists with respect to theoretical calculations in the value of the energy shift of the absorption onset and so this comparison should help to refine the approximations used in models.

A microsecond time resolved x-ray absorption near edge structure synchrotron study of phase transitions in Fe undergoing ramp heating at high pressure

We report a microsecond time-resolved x-ray absorption near edge structure study using synchrotron radiation to dynamically detect structural phase transitions in Fe undergoing rapid heating along a quasi-isochoric path. Within a few ms, we observed two structural phase transitions, which transform the ambient bcc phase of Fe into the fcc phase, and then into the liquid phase. This example illustrates the opportunities offered by energy dispersive x-ray absorption spectroscopy in the study of matter under extreme dynamic conditions. Advanced simulations are compared to these data.

Taking Thin Diamonds to Their Limit: Coupling Static‐Compression and Laser‐Shock Techniques to Generate Dense Water

Laser‐driven Hugoniot experiments on precompressed samples access thermodynamic conditions unreachable by either static or single‐shock compression techniques alone. Recent experiments using Rutherford Appleton Laboratory’s Vulcan Laser achieved final pressures up to ∼200 GPa and temperatures up to 10,000 K in water samples precompressed to ∼1 GPa, thereby validating this new technique. Diamond anvils, used for sample precompression, must be thin in order to avoid rarefaction catch up; but thin diamonds fail under pressure. Anvils no more than 200 μm thick were encased in diamond cells modified to accommodate the laser‐beam geometry.

The α → ω phase transformation in zirconium followed with ms-scale time-resolved X-ray absorption spectroscopy

ABSTRACT The conditions of the pressure-induced phase transformation in zirconium have been reported to be influenced by the sample purity, the pressurizing conditions, and the deformation rate. Here, we study this transformation using a dynamic diamond-anvil cell compression setup and ms-scale time-resolved X-ray absorption spectroscopy. The sample pressure is also monitored at the same timescale using the ruby luminescence method. For the samples used in this study (polycrystal of 99.9+ purity, hydrostatically compressed in neon pressure transmitting medium), the phase transformation pressure is very close in static (11 GPa) and in dynamic (12±1 GPa) compression and the transformation is achieved in less than a few ms at 12 GPa. Comparison with literature studies suggest that the kinetics and the mechanism of this martensitic phase transformation are different under hydrostatic and non-hydrostatic compression.

A symmetric miniature diamond anvil cell for magnetic measurements on dense hydrides in a SQUID magnetometer

ABSTRACT A new miniature diamond anvil cell was specifically designed to detect superconductivity using a SQUID (Superconducting QUantum Interference Device) magnetometer in dense hydrides directly synthesized by the reaction of hydrogen with a chemical element. The cell, made of a CuTi alloy, is fully symmetric with a very low magnetic background allowing the detection of the superconductivity of a sample as small as 3.4 × 104 µm3 without background subtraction. DC measurements or AC measurements in a Magnetic Property Measurement System 3 SQUID magnetometer from Quantum Design could be performed at temperatures as low as 3 K. This high pressure cell is inserted in a modified conventional membrane diamond anvil cell to be driven for hydrogen gas loading and for fine pressure increase before magnetic measurements are performed. To synthetize and structurally characterize the superconducting sample, a 21° optical and 8.6° X-ray acceptance angle allows one to perform laser heating and X-ray diffraction at the same time. A first measurement is shown on the PdH system.

High pressure dynamic XAS studies using an energy-dispersive spectrometer

ABSTRACT We present in this paper recent advances in the high pressure domain provided by the introduction of time-resolved energy-dispersive XAS (EDXAS) techniques at synchrotrons. We highlight technical aspects and describe two modes of acquisition: the ‘movie’ mode, where the time resolution is given by the detector acquisition speed and the ‘pump-and-probe’ mode, where the time resolution is given by the delay between the pump and the probe. These two modes define a frontier in the time resolution, respectively above and below the ∼10 μs regime. In the former, examples of applications are chemical stability and reactions at high pressure and high temperature or probing the warm dense matter regime using rapid current ramps. In the latter, an example is given on studies of dynamically compressed matter, by coupling single-bunch EDXAS at high-brilliance synchrotron to a nanosecond high-power laser.

MICROSTRUCTURAL INVESTIGATION OF LASER‐SHOCKED IRON FOILS

High‐power laser shots were carried out on 100 microns thick iron foils. The present paper is devoted to the study of recovered fragments. Analysis of the fragments morphology shows the influence of shock pressure on the fragmentation process. Evaluation of the fragment‐size distribution is performed as a function of the laser power and the measured free surface velocity. The results highlight a clear evolution with laser power density.