Optical full-field strain measurement within a diamond anvil cell.

Digital image correlation computations are run on optical images of iron samples within a diamond anvil cell to obtain in-plane strain components at the surface of the sample up to 17 GPa. The α-Fe → ε-Fe transition onset pressure and phase coexistence pressure domain can be identified from the evolution of the surface average of strain components. Strain fields exhibit localizations for both direct and reverse transition; they coincide with the approximate boundary locations of reversion variants inside the microstructure of a single crystal sample. The so-called DICDAC (Digital Image Correlation within a Diamond Anvil Cell) setup is then a suitable tool for the investigation of phase transformations strains under pressure. In addition, specific volumes that are deduced from strain out of the transition pressure domains agree within ΔV/V = 0.4% with the equation of state data from the literature.

Synthesis of Single Crystals of ε-Iron and Direct Measurements of Its Elastic Constants.

Seismology finds that Earth's solid inner core behaves anisotropically. Interpretation of this requires a knowledge of crystalline elastic anisotropy of its constituents-the major phase being most likely ε-Fe, stable only under high pressure. Here, single crystals of this phase are synthesized, and its full elasticity tensor is measured between 15 and 33 GPa at 300 K. It is calculated under the same conditions, using the combination of density functional theory and dynamical mean field theory, which describes explicitly electronic correlation effects. The predictive power of this scheme is checked by comparison with measurements; it is then used to evaluate the crystalline anisotropy in ε-Fe under higher density. This anisotropy remains of the same amplitude up to densities typical of Earth's inner core.

Stability and equation of state of face-centered cubic and hexagonal close packed phases of argon under pressure.

The compression of argon is measured between 10 K and 296 K up to 20 GPa and and up to 114 GPa at 296 K in diamond anvil cells. Three samples conditioning are used: (1) single crystal sample directly compressed between the anvils, (2) powder sample directly compressed between the anvils, (3) single crystal sample compressed in a pressure medium. A partial transformation of the face-centered cubic (fcc) phase to a hexagonal close-packed (hcp) structure is observed above 4.2-13 GPa. Hcp phase forms through stacking faults in fcc-Ar and its amount depends on pressurizing conditions and starting fcc-Ar microstructure. The quasi-hydrostatic equation of state of the fcc phase is well described by a quasi-harmonic Mie-Grüneisen-Debye formalism, with the following 0 K parameters for Rydberg-Vinet equation: [Formula: see text] = 38.0 Å[Formula: see text]/at, [Formula: see text] = 2.65 GPa, [Formula: see text] = 7.423. Under the current experimental conditions, non-hydrostaticity affects measured P-V points mostly at moderate pressure ([Formula: see text] 20 GPa).

Argon-neon binary diagram and ArNe2 Laves phase.

Mixtures of argon and neon have been experimentally studied under high pressure. One stoichiometric compound, with ArNe2 composition, is observed in this system. It is a Laves phase with a hexagonal MgZn2 structure, stable up to at least 65 GPa, the highest pressure reached in the experiments. Its equation of state follows closely the one of an ideal Ar+2Ne mixture. The binary phase diagram of the Ar-Ne system resembles the diagram predicted for hard sphere mixtures with a similar atomic radius ratio, suggesting that no electronic interactions appear in this system in this pressure range. ArNe2 can be a convenient quasihydrostatic pressure transmitting medium under moderate pressure.

In situ characterization of the high pressure - high temperature melting curve of platinum.

In this work, the melting line of platinum has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using ab initio simulations. In the investigated pressure and temperature range (pressure between 10 GPa and 110 GPa and temperature between 300 K and 4800 K), only the face-centered cubic phase of platinum has been observed. The melting points obtained with the two techniques are in good agreement. Furthermore, the obtained results agree and considerably extend the melting line previously obtained in large-volume devices and in one laser-heated diamond-anvil cells experiment, in which the speckle method was used as melting detection technique. The divergence between previous laser-heating experiments is resolved in favor of those experiments reporting the higher melting slope.

Toroidal diamond anvil cell for detailed measurements under extreme static pressures.

Over the past 60 years, the diamond anvil cell (DAC) has been developed into a widespread high static pressure device. The adaptation of laboratory and synchrotron analytical techniques to DAC enables a detailed exploration in the 100 GPa range. The strain of the anvils under high load explains the 400 GPa limit of the conventional DAC. Here we show a toroidal shape for a diamond anvil tip that enables to extend the DAC use toward the terapascal pressure range. The toroidal-DAC keeps the assets for a complete, reproducible, and accurate characterization of materials, from solids to gases. Raman signal from the diamond anvil or X-ray signal from the rhenium gasket allow measurement of pressure. Here, the equations of state of gold, aluminum, and argon are measured with X-ray diffraction. The data are compared with recent measurements under similar conditions by two other approaches, the double-stage DAC and the dynamic ramp compression.

High Pressure and High Temperature Synthesis of the Iron Pernitride FeN2.

The high pressure chemistry of transition metals and nitrogen was recently discovered to be richer than previously thought, due to the synthesis of several transition metal pernitrides. Here, we explore the pressure-temperature domain of iron with an excess of nitrogen up to 91 GPa and 2200 K. Above 72 GPa and 2200 K, the iron pernitride FeN2 is produced in a laser-heated diamond anvil cell. This iron-nitrogen compound is the first with a N/Fe ratio greater than 1. The FeN2 samples were characterized from the maximum observed pressure down to ambient conditions by powder X-ray diffraction and Raman spectroscopy measurements. The crystal structure of FeN2 is resolved to be a Pnnm marcasite structure, analogously to other transition metal pernitrides. On the basis of the lattice's axial ratios and the recorded N-N vibrational modes of FeN2, a bond order of 1.5 for the nitrogen dimer is suggested. The bulk modulus of the iron pernitride is determined to be of K0 = 344(13) GPa, corresponding to an astounding increase of about 208% from pure iron. Upon decompression to ambient conditions, a partial structural phase transition to the theoretically predicted R3̅ m FeN2 is detected.

Synthesis and stability of xenon oxides Xe2O5 and Xe3O2 under pressure.

The noble gases are the most inert group of the periodic table, but their reactivity increases with pressure. Diamond-anvil-cell experiments and ab initio modelling have been used to investigate a possible direct reaction between xenon and oxygen at high pressures. We have now synthesized two oxides below 100 GPa (Xe2O5 under oxygen-rich conditions, and Xe3O2 under oxygen-poor conditions), which shows that xenon is more reactive under pressure than predicted previously. Xe2O5 was observed using X-ray diffraction methods, its structure identified through ab initio random structure searching and confirmed using X-ray absorption and Raman spectroscopies. The experiments confirm the recent prediction of Xe3O2 as a stable xenon oxide under high pressure. Xenon atoms adopt mixed oxidation states of 0 and +4 in Xe3O2 and +4 and +6 in Xe2O5. Xe3O2 and Xe2O5 form extended networks that incorporate oxygen-sharing XeO4 squares, and Xe2O5 additionally incorporates oxygen-sharing XeO5 pyramids. Other xenon oxides (XeO2, XeO3) are expected to form at higher pressures.

New iron hydrides under high pressure.

The Fe-H system has been investigated by combined x-ray diffraction studies and total energy calculations at pressures up to 136 GPa. The experiments involve laser annealing of hydrogen-embedded iron in a diamond anvil cell. Two new FeHx compounds, with x∼2 and x=3, are discovered at 67 and 86 GPa, respectively. Their crystal structures are identified (unit cell and Fe positional parameters from x-ray diffraction, H positional parameters from ab initio calculations) as tetragonal with space group I4/mmm for FeH(∼2) and as simple cubic with space group Pm3m for FeH3. Large metastability regimes are observed that allowed to measure the P(V) equation of state at room temperature of FeH, FeH(∼2), and FeH3.

High melting points of tantalum in a laser-heated diamond anvil cell.

In situ x-ray diffraction has been used to characterize the structural modifications of tantalum samples under intense laser irradiation, up to 135 GPa in a diamond anvil cell. Melting data points are obtained that do not confirm the previously reported anomalously low melting curve. Two effects are identified that might alter the melting determination of refractory metals such as Ta under high static pressures. First, a strong chemical reactivity of Ta with the pressure transmitting media and with carbon diffusing out from the surface of the anvils is observed. Second, pyrometry measurements can be distorted when the pressure medium melts. The strong divergence between ab initio calculations, shock measurements and static determination is resolved here and hence many theoretical interpretations are ruled out. Finally, the body-centered cubic phase is stable over the pressure-temperature range investigated.

Quasihydrostatic equation of state of iron above 2 Mbar.

The compression curve of iron is measured up to 205 GPa at 298 K, under quasihydrostatic conditions in a diamond anvil cell. Above 150 GPa, the compression of this metal is significantly higher than previously measured under nonhydrostatic conditions. The same compression curve is also calculated ab initio and the deviation between experiment and theory is clearly established. A formulation of the equation of state of iron over a large pressure and temperature range, based on the current data and existing shock-wave data, is also proposed. Implications for the Earth's core are discussed.

Melting curve and fluid equation of state of carbon dioxide at high pressure and high temperature.

The melting curve and fluid equation of state of carbon dioxide have been determined under high pressure in a resistively heated diamond anvil cell. The melting line was determined from room temperature up to 11.1+/-0.1 GPa and 800+/-5 K by visual observation of the solid-fluid equilibrium and in situ measurements of pressure and temperature. Raman spectroscopy was used to identify the solid phase in equilibrium with the melt, showing that solid I is the stable phase along the melting curve in the probed range. Interferometric and Brillouin scattering experiments were conducted to determine the refractive index and sound velocity of the fluid phase. A dispersion of the sound velocity between ultrasonic and Brillouin frequencies is evidenced and could be reproduced by postulating the presence of a thermal relaxation process. The Brillouin sound velocities were then transformed to thermodynamic values in order to calculate the equation of state of fluid CO2. An analytic formulation of the density with respect to pressure and temperature is proposed, suitable in the P-T range of 0.1-8 GPa and 300-700 K and accurate within 2%. Our results show that the fluid above 500 K is less compressible than predicted from various phenomenological models.