Sound velocities and thermodynamical properties of hcp iron at high pressure and temperature.

Sound velocities and thermodynamical properties of hcp iron have been computed usingab initiocalculations over an extended density and temperature range, encompassing the conditions directly relevant for the Earth's inner core. At room temperature, and up to 350 GPa, an excellent agreement is obtained between present results and experimental data for many thermodynamical quantities: phonon density of states, vibrational entropy, heat capacity, Grüneisen parameter and thermal expansion. With increasing temperature, along an isochore, we observe a strong decrease of the phonon frequencies, demonstrating that intrinsic anharmonic effects cannot be neglected. We also carefully compare previous theoretical data for the sound velocities and try to explain the discrepancies observed with experiments. Finally, we propose a temperature dependant Birch's law that we compare with previous experimental work.

The Fe-FeSi phase diagram at Mercury's core conditions.

Mercury's metallic core is expected to have formed under highly reducing conditions, resulting in the presence of significant quantities of silicon alloyed to iron. Here we present the phase diagram of the Fe-FeSi system, reconstructed from in situ X-ray diffraction measurements at pressure and temperature conditions spanning over those expected for Mercury's core, and ex situ chemical analysis of recovered samples. Under high pressure, we do not observe a miscibility gap between the cubic fcc and B2 structures, but rather the formation of a re-entrant bcc phase at temperatures close to melting. Upon melting, the investigated alloys are observed to evolve towards two distinct Fe-rich and Fe-poor liquid compositions at pressures below 35-38 GPa. The evolution of the phase diagram with pressure and temperature prescribes a range of possible core crystallization regimes, with strong dependence on the Si abundance of the core.

Diffusionless γ⇄α phase transition in polycrystalline and single-crystal cerium.

The cerium γ⇄α transition was investigated using high-pressure, high-temperature angle-dispersive x-ray diffraction measurements on both poly- and single-crystalline samples, explicitly addressing symmetry change and transformation paths. The isomorphic hypothesis of the transition is confirmed, with a transition line ending at a solid-solid critical point. The critical exponent is determined, showing a universal behavior that can be pictured as a liquid-gas transition. We further report an isomorphic transition between two single crystals (with more than 14% of volume difference), an unparalleled observation in solid-state matter interpreted in terms of dislocation-induced diffusionless first-order phase transformation.

3D imaging of the Fermi surface by thermal diffuse scattering.

We use thermal diffuse scattering of x rays to visualize the lens-shaped portions of the Fermi surface in metallic zinc. Our interpretation of the nature of the observed scattered intensity anomalies is supported by the incorporation of inelastic x-ray scattering measurements as well as ab initio calculations of the electronic structure and lattice dynamics. Our work demonstrates that thermal diffuse scattering complements well-established techniques and is a powerful tool in its own right for studying the shape of the Fermi surface through the associated electron-phonon coupling.

Elasticity of cobalt at high pressure studied by inelastic x-ray scattering.

The five independent elastic moduli of single-crystalline hcp cobalt were determined by inelastic x-ray scattering to 39 GPa and compared to ultrasonic measurements and first principles calculations. In general the agreement is good, in particular, for the evolution of the longitudinal sound velocity in the a-c plane. This confirms the calculations, suggesting that a similar evolution is valid for hcp iron, the main constituent of the Earth's inner core, up to the highest investigated pressure. Our results represent an important benchmark to further refine ab initio calculations.